Add support for monitoring gpio switches

This driver was written to expose a read only interface to a number of
gpios on Chromebooks. These gpios are attached to signals which cause the
firmware on Chromebooks to enter alternative modes of operation and/or
control other device characteristics (such as write protection on flash
devices). It was originally posted as "Add support for monitoring Chrome
OS firmware signals". A request was made to make it more generic.

In addition this patch series provides the required bindings for this to
the peach-pi Chromebook.

This is a new binding, but the driver is based (now some what loosely) on
functionality in the kernel shipped on Chromebooks.

This patch adds documentation for the gpio-switch binding. This binding
provides a mechanism to bind named links to gpio, with the primary
purpose of enabling standardised access to switches that might be standard
across a group of devices but implemented differently on each device.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
---
 .../devicetree/bindings/misc/gpio-switch.txt       | 47 ++++++++++++++++++++++
 1 file changed, 47 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/misc/gpio-switch.txt

diff --git a/Documentation/devicetree/bindings/misc/gpio-switch.txt b/Documentation/devicetree/bindings/misc/gpio-switch.txt
new file mode 100644
index 0000000..13528bd
--- /dev/null
+++ b/Documentation/devicetree/bindings/misc/gpio-switch.txt

@@ -0,0 +1,47 @@
+Device-Tree bindings for gpio attached switches.
+
+This provides a mechanism to provide a named link to specified gpios. This can
+be useful in instances such as when theres a need to monitor a switch, which is
+common across a family of devices, but attached to different gpios and even
+implemented in different ways on differnet devices.
+
+Required properties:
+	- compatible = "gpio-switch";
+
+Each signal is represented as a sub-node of "gpio-switch". The naming of
+sub-nodes is arbitrary.
+
+Required sub-node properties:
+
+	- label: Name to be given to gpio switch.
+	- gpios: OF device-tree gpio specification.
+
+Optional sub-node properties:
+
+	- read-only: Boolean flag to mark the gpio as read-only, i.e. the line
+	  should not be driven by the host.
+
+Example nodes:
+
+        gpio-switch {
+                compatible = "gpio-switch";
+
+                write-protect {
+                        label = "write-protect";
+                        gpios = <&gpx3 0 GPIO_ACTIVE_LOW>;
+                        read-only;
+                };
+
+                developer-switch {
+                        label = "developer-switch";
+                        gpios = <&gpx1 3 GPIO_ACTIVE_HIGH>;
+                        read-only;
+                };
+
+                recovery-switch {
+                        label = "recovery-switch";
+                        gpios = <&gpx0 7 GPIO_ACTIVE_LOW>;
+                        read-only;
+                };
+        };
+

-- 
2.1.4

The peach pi has a GPIO connected to the firmware write protect, developer
mode and recovery mode lines (which are primarily controlled via external
switches on developer test board). This patch adds the required nodes to
the device tree to configure the pinmuxing and allow these to be read from
user space.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
---
 arch/arm/boot/dts/exynos5800-peach-pi.dts | 46 +++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/arch/arm/boot/dts/exynos5800-peach-pi.dts b/arch/arm/boot/dts/exynos5800-peach-pi.dts
index 49a4f43..2937372 100644
--- a/arch/arm/boot/dts/exynos5800-peach-pi.dts
+++ b/arch/arm/boot/dts/exynos5800-peach-pi.dts

@@ -53,6 +53,31 @@
 		};
 	};
 
+	gpio-switch {
+		compatible = "gpio-switch";
+
+		pinctrl-names = "default";
+		pinctrl-0 = <&wp_gpio &dev_mode &rec_mode>;
+
+		write-protect {
+			label = "write-protect";
+			gpios = <&gpx3 0 GPIO_ACTIVE_LOW>;
+			read-only;
+		};
+
+		developer-switch {
+			label = "developer-switch";
+			gpios = <&gpx1 3 GPIO_ACTIVE_HIGH>;
+			read-only;
+		};
+
+		recovery-switch {
+			label = "recovery-switch";
+			gpios = <&gpx0 7 GPIO_ACTIVE_LOW>;
+			read-only;
+		};
+	};
+
 	gpio-keys {
 		compatible = "gpio-keys";
 

@@ -731,6 +756,13 @@
 		samsung,pin-val = <0>;
 	};
 
+	rec_mode: rec-mode {
+		samsung,pins = "gpx0-7";
+		samsung,pin-function = <0>;
+		samsung,pin-pud = <0>;
+		samsung,pin-drv = <0>;
+	};
+
 	tpm_irq: tpm-irq {
 		samsung,pins = "gpx1-0";
 		samsung,pin-function = <0>;

@@ -752,6 +784,13 @@
 		samsung,pin-drv = <0>;
 	};
 
+	dev_mode: dev-mode {
+		samsung,pins = "gpx1-3";
+		samsung,pin-function = <0>;
+		samsung,pin-pud = <3>;
+		samsung,pin-drv = <0>;
+	};
+
 	ec_irq: ec-irq {
 		samsung,pins = "gpx1-5";
 		samsung,pin-function = <0>;

@@ -773,6 +812,13 @@
 		samsung,pin-drv = <0>;
 	};
 
+	wp_gpio: wp_gpio {
+		samsung,pins = "gpx3-0";
+		samsung,pin-function = <0>;
+		samsung,pin-pud = <0>;
+		samsung,pin-drv = <0>;
+	};
+
 	max77802_irq: max77802-irq {
 		samsung,pins = "gpx3-1";
 		samsung,pin-function = <0>;

-- 
2.1.4

Select Chromebooks have gpio attached to switches used to cause the
firmware to enter alternative modes of operation and/or control other
device characteristics (such as write protection on flash devices). This
patch adds a driver that exposes a read-only interface to allow these
signals to be read from user space.

This functionality has been generalised to provide support for any device
with device tree support which needs to identify a gpio as being used for a
specific task.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
---
 drivers/misc/Kconfig       |  11 ++++
 drivers/misc/Makefile      |   1 +
 drivers/misc/gpio-switch.c | 160 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 172 insertions(+)
 create mode 100644 drivers/misc/gpio-switch.c

diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
index 22892c7..d24367c 100644
--- a/drivers/misc/Kconfig
+++ b/drivers/misc/Kconfig

@@ -525,6 +525,17 @@ config VEXPRESS_SYSCFG
 	  bus. System Configuration interface is one of the possible means
 	  of generating transactions on this bus.
 
+config GPIO_SWITCH
+	tristate "GPIO Switch driver"
+	depends on GPIO_SYSFS
+	---help---
+	 Some devices have gpio attached to dedicated switches, an example of
+	 this are chromebooks (where connection to some switches for predefined
+	 purposes are provided on the generic development card). This driver
+	 provides the ability to create consistently named sysfs entries to the
+	 functionally equivalent signals across a range of devices. This
+	 functionality currently requires a device which supports device tree.
+
 source "drivers/misc/c2port/Kconfig"
 source "drivers/misc/eeprom/Kconfig"
 source "drivers/misc/cb710/Kconfig"

diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
index 537d7f3..7a7e11a 100644
--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile

@@ -56,3 +56,4 @@ obj-$(CONFIG_GENWQE)		+= genwqe/
 obj-$(CONFIG_ECHO)		+= echo/
 obj-$(CONFIG_VEXPRESS_SYSCFG)	+= vexpress-syscfg.o
 obj-$(CONFIG_CXL_BASE)		+= cxl/
+obj-$(CONFIG_GPIO_SWITCH)	+= gpio-switch.o

diff --git a/drivers/misc/gpio-switch.c b/drivers/misc/gpio-switch.c
new file mode 100644
index 0000000..1f381d6
--- /dev/null
+++ b/drivers/misc/gpio-switch.c

@@ -0,0 +1,160 @@
+/*
+ * Copyright (C) 2015 Collabora Ltd.
+ *
+ * based on vendor driver,
+ *
+ * Copyright (C) 2011 The Chromium OS Authors
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+#include <linux/bcd.h>
+#include <linux/gpio.h>
+#include <linux/notifier.h>
+#include <linux/io.h>
+#include <linux/of.h>
+#include <linux/of_gpio.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+
+struct gpio_switch_gpio_info {
+	int gpio;
+	const char *link;
+};
+
+static int dt_gpio_init(struct platform_device *pdev, struct device_node *child,
+			struct gpio_switch_gpio_info *gpio)
+{
+	int err;
+	enum of_gpio_flags of_flags;
+	unsigned long flags = GPIOF_DIR_IN | GPIOF_EXPORT;
+	const char *name;
+
+	err = of_property_read_string(child, "label", &name);
+	if (err)
+		return err;
+
+	gpio->gpio = of_get_named_gpio_flags(child, "gpios", 0, &of_flags);
+	if (!gpio_is_valid(gpio->gpio)) {
+		err = -EINVAL;
+		goto err_prop;
+	}
+
+	if (of_flags & OF_GPIO_ACTIVE_LOW)
+		flags |= GPIOF_ACTIVE_LOW;
+
+	if (!of_property_read_bool(child, "read-only"))
+		flags |= GPIOF_EXPORT_CHANGEABLE;
+
+	err = gpio_request_one(gpio->gpio, flags, name);
+	if (err)
+		goto err_prop;
+
+	err = gpio_export_link(&pdev->dev, name, gpio->gpio);
+	if (err)
+		goto err_gpio;
+
+	gpio->link = name;
+
+	return 0;
+
+err_gpio:
+	gpio_free(gpio->gpio);
+err_prop:
+	of_node_put(child);
+
+	return err;
+}
+
+static void gpio_switch_rem(struct device *dev,
+			    struct gpio_switch_gpio_info *gpio)
+{
+	sysfs_remove_link(&dev->kobj, gpio->link);
+
+	gpio_unexport(gpio->gpio);
+
+	gpio_free(gpio->gpio);
+}
+
+static int gpio_switch_probe(struct platform_device *pdev)
+{
+	struct gpio_switch_gpio_info *gpios;
+	struct device_node *child;
+	struct device_node *np = pdev->dev.of_node;
+	int ret;
+	int i;
+
+	i = of_get_child_count(np);
+	if (i < 1)
+		return i;
+
+	gpios = devm_kmalloc(&pdev->dev, sizeof(gpios) * i, GFP_KERNEL);
+	if (!gpios)
+		return -ENOMEM;
+
+	i = 0;
+
+	for_each_child_of_node(np, child) {
+		ret = dt_gpio_init(pdev, child, &gpios[i]);
+		if (ret) {
+			dev_err(&pdev->dev, "Failed to init child node %d.\n",
+				i);
+			goto err;
+		}
+
+		i++;
+	}
+
+	platform_set_drvdata(pdev, gpios);
+
+	return 0;
+
+err:
+	while (i > 0) {
+		i--;
+		gpio_switch_rem(&pdev->dev, &gpios[i]);
+	}
+
+	return ret;
+}
+
+static int gpio_switch_remove(struct platform_device *pdev)
+{
+	struct gpio_switch_gpio_info *gpios = platform_get_drvdata(pdev);
+	struct device_node *np = pdev->dev.of_node;
+	int i;
+
+	i = of_get_child_count(np);
+
+	while (i > 0) {
+		i--;
+		gpio_switch_rem(&pdev->dev, &gpios[i]);
+	}
+
+	return 0;
+}
+
+static const struct of_device_id gpio_switch_of_match[] = {
+	{ .compatible = "gpio-switch" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, gpio_switch_of_match);
+
+static struct platform_driver gpio_switch_driver = {
+	.probe = gpio_switch_probe,
+	.remove = gpio_switch_remove,
+	.driver = {
+		.name = "gpio_switch",
+		.of_match_table = gpio_switch_of_match,
+	},
+};
+module_platform_driver(gpio_switch_driver);
+
+MODULE_LICENSE("GPL");

-- 
2.1.4

Hi Martyn,

[auto build test WARNING on char-misc/char-misc-testing]
[also build test WARNING on v4.4-rc3 next-20151203]

url:    https://github.com/0day-ci/linux/commits/Martyn-Welch/Device-tree-binding-documentation-for-gpio-switch/20151205-014105


coccinelle warnings: (new ones prefixed by >>)

quoted

drivers/misc/gpio-switch.c:98:34-40: ERROR: application of sizeof to pointer

Please review and possibly fold the followup patch.

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

drivers/misc/gpio-switch.c:98:34-40: ERROR: application of sizeof to pointer

 sizeof when applied to a pointer typed expression gives the size of
 the pointer

Generated by: scripts/coccinelle/misc/noderef.cocci

CC: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Fengguang Wu <redacted>
---

 gpio-switch.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/misc/gpio-switch.c
+++ b/drivers/misc/gpio-switch.c

@@ -95,7 +95,7 @@ static int gpio_switch_probe(struct plat
 	if (i < 1)
 		return i;
 
-	gpios = devm_kmalloc(&pdev->dev, sizeof(gpios) * i, GFP_KERNEL);
+	gpios = devm_kmalloc(&pdev->dev, sizeof(*gpios) * i, GFP_KERNEL);
 	if (!gpios)
 		return -ENOMEM;
 

On Fri, Dec 04, 2015 at 05:31:14PM +0000, Martyn Welch wrote:

Select Chromebooks have gpio attached to switches used to cause the
firmware to enter alternative modes of operation and/or control other
device characteristics (such as write protection on flash devices). This
patch adds a driver that exposes a read-only interface to allow these
signals to be read from user space.

This functionality has been generalised to provide support for any device
with device tree support which needs to identify a gpio as being used for a
specific task.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
---
 drivers/misc/Kconfig       |  11 ++++
 drivers/misc/Makefile      |   1 +
 drivers/misc/gpio-switch.c | 160 +++++++++++++++++++++++++++++++++++++++++++++

Why isn't this in drivers/gpio/ ?

why make it a misc driver?

thanks,

greg k-h

On 04/12/15 18:57, Greg Kroah-Hartman wrote:

On Fri, Dec 04, 2015 at 05:31:14PM +0000, Martyn Welch wrote:

quoted

Select Chromebooks have gpio attached to switches used to cause the
firmware to enter alternative modes of operation and/or control other
device characteristics (such as write protection on flash devices). This
patch adds a driver that exposes a read-only interface to allow these
signals to be read from user space.

This functionality has been generalised to provide support for any device
with device tree support which needs to identify a gpio as being used for a
specific task.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
---
  drivers/misc/Kconfig       |  11 ++++
  drivers/misc/Makefile      |   1 +
  drivers/misc/gpio-switch.c | 160 +++++++++++++++++++++++++++++++++++++++++++++

Why isn't this in drivers/gpio/ ?

why make it a misc driver?

I thought all the drivers in /drivers/gpio were gpio drivers, rather 
than users of the gpio framework. Is that not the case?

Happy to move it if the consensus is that that's the correct place to 
put it.

Martyn

thanks,

greg k-h

+Linus W

On Fri, Dec 04, 2015 at 05:31:13PM +0000, Martyn Welch wrote:

This patch adds documentation for the gpio-switch binding. This binding
provides a mechanism to bind named links to gpio, with the primary
purpose of enabling standardised access to switches that might be standard
across a group of devices but implemented differently on each device.

This is good and what I suggested, but it now makes me wonder if switch 
is generic enough. This boils down to needing to expose single gpio 
lines to userspace with a defined function/use. IIRC, there's been some 
discussion about this before along with improving the userspace 
interface for GPIO in general. So I'd like to get Linus' thoughts on 
this.

In terms a a switch use, allowing driving it would be an override of the 
switch. Is that the idea here?

+
+Example nodes:
+
+        gpio-switch {
+                compatible = "gpio-switch";

Both from a binding and driver perspective, there is no point in 
grouping these. Each node can simply have this compatible string.

+
+                write-protect {
+                        label = "write-protect";
+                        gpios = <&gpx3 0 GPIO_ACTIVE_LOW>;
+                        read-only;
+                };
+
+                developer-switch {
+                        label = "developer-switch";
+                        gpios = <&gpx1 3 GPIO_ACTIVE_HIGH>;
+                        read-only;
+                };
+
+                recovery-switch {
+                        label = "recovery-switch";
+                        gpios = <&gpx0 7 GPIO_ACTIVE_LOW>;
+                        read-only;
+                };
+        };
+
-- 
2.1.4

On 07/12/15 17:37, Rob Herring wrote:

+Linus W

On Fri, Dec 04, 2015 at 05:31:13PM +0000, Martyn Welch wrote:

quoted

This patch adds documentation for the gpio-switch binding. This binding
provides a mechanism to bind named links to gpio, with the primary
purpose of enabling standardised access to switches that might be standard
across a group of devices but implemented differently on each device.

This is good and what I suggested, but it now makes me wonder if switch
is generic enough. This boils down to needing to expose single gpio
lines to userspace with a defined function/use. IIRC, there's been some
discussion about this before along with improving the userspace
interface for GPIO in general. So I'd like to get Linus' thoughts on
this.

No problem. Rename gpio-signal?

quoted

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
---
  .../devicetree/bindings/misc/gpio-switch.txt       | 47 ++++++++++++++++++++++
  1 file changed, 47 insertions(+)
  create mode 100644 Documentation/devicetree/bindings/misc/gpio-switch.txt

diff --git a/Documentation/devicetree/bindings/misc/gpio-switch.txt b/Documentation/devicetree/bindings/misc/gpio-switch.txt
new file mode 100644
index 0000000..13528bd
--- /dev/null
+++ b/Documentation/devicetree/bindings/misc/gpio-switch.txt

@@ -0,0 +1,47 @@
+Device-Tree bindings for gpio attached switches.
+
+This provides a mechanism to provide a named link to specified gpios. This can
+be useful in instances such as when theres a need to monitor a switch, which is
+common across a family of devices, but attached to different gpios and even
+implemented in different ways on differnet devices.
+
+Required properties:
+	- compatible = "gpio-switch";
+
+Each signal is represented as a sub-node of "gpio-switch". The naming of
+sub-nodes is arbitrary.
+
+Required sub-node properties:
+
+	- label: Name to be given to gpio switch.
+	- gpios: OF device-tree gpio specification.
+
+Optional sub-node properties:
+
+	- read-only: Boolean flag to mark the gpio as read-only, i.e. the line
+	  should not be driven by the host.

In terms a a switch use, allowing driving it would be an override of the
switch. Is that the idea here?

Yeah - since it had become a lot more generic and a lot of 
switches/signals would probably be implemented with a pull-up resistor 
of something like that, it seemed to make sense to allow them to be 
driven as well.

quoted

+
+Example nodes:
+
+        gpio-switch {
+                compatible = "gpio-switch";

Both from a binding and driver perspective, there is no point in
grouping these. Each node can simply have this compatible string.

True. I did it this way as this is how gpio-keys is implemented. OK, 
that has one optional parameter (autorepeat) for the block and this has 
none. Though I can also see that these probably have less in common than 
the individual lines used in gpio-keys.

quoted

+
+                write-protect {
+                        label = "write-protect";
+                        gpios = <&gpx3 0 GPIO_ACTIVE_LOW>;
+                        read-only;
+                };
+
+                developer-switch {
+                        label = "developer-switch";
+                        gpios = <&gpx1 3 GPIO_ACTIVE_HIGH>;
+                        read-only;
+                };
+
+                recovery-switch {
+                        label = "recovery-switch";
+                        gpios = <&gpx0 7 GPIO_ACTIVE_LOW>;
+                        read-only;
+                };
+        };
+
--
2.1.4

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

Select Chromebooks have gpio attached to switches used to cause the
firmware to enter alternative modes of operation and/or control other
device characteristics (such as write protection on flash devices). This
patch adds a driver that exposes a read-only interface to allow these
signals to be read from user space.

This functionality has been generalised to provide support for any device
with device tree support which needs to identify a gpio as being used for a
specific task.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>

If you want to do this thing, also propose a device tree binding document
for "gpio-switch".

But first (from Documentation/gpio/drivers-on-gpio.txt):

- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
  can generate interrupts in response to a key press. Also supports debounce.

- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
  GPIO line cannot generate interrupts, so it needs to be periodically polled
  by a timer.

- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
  external connector status, such as a headset line for an audio driver or an
  HDMI connector. It will provide a better userspace sysfs interface than GPIO.

So you mean none of these apply for this case?

Second: what you want to do is export a number of GPIOs with certain names
to userspace. This is something very generic and should be implemented
as such, not as something Chromebook-specific.

Patches like that has however already been suggested, and I have NACKed
them because the GPIO sysfs ABI is insane, and that is why I am refactoring
the world to create a proper chardev ABI for GPIO instead. See:
http://marc.info/?l=linux-gpio&m=144550276512673&w=2

So for the moment, NACK on this, please participate in creating the
*right* ABI for GPIO instead of trying to shoehorn stuff into the dying
sysfs ABI.

Yours,
Linus Walleij

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

This patch adds documentation for the gpio-switch binding. This binding
provides a mechanism to bind named links to gpio, with the primary
purpose of enabling standardised access to switches that might be standard
across a group of devices but implemented differently on each device.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>

As mentioned in the comment to the second patch, this solves the
following generic problem:

Expose a GPIO line to userspace using a specific name

That means basically naming GPIO lines and marking them as
"not used by the operating system".

This is something that has been proposed before, and postponed
because the kernel lacks the right infrastructure.

Markus Pargmann also did a series that add initial values to
hogs, which is the inverse usecase of this, where you want to
*output* something by default, then maybe also make it available
to userspace.

So what we need to see here is a patch series that does all of these
things:

- Name lines

- Sets them to initial values

- Mark them as read-only

- Mark them as "not used by the operating system" so that they
  can be default-exported to userspace.

Making *USE* of this naming etc inside the Linux kernel

+        gpio-switch {
+                compatible = "gpio-switch";
+
+                write-protect {
+                        label = "write-protect";
+                        gpios = <&gpx3 0 GPIO_ACTIVE_LOW>;
+                        read-only;
+                };

This should not need new structures and nodes like this. It should
be part of Documentation/devicetree/bindings/gpio/gpio.txt
and put directly in the gpiochip node.

Maybe as an extension of the existing hogs, but that has already
been tried.

While we can agree on a device tree binding, the kernel still needs
major refactoring to actually expose named GPIOs to userspace,
and that should be done using the new chardev, not with sysfs
links.

Yours,
Linus Walleij

On Fri, Dec 11, 2015 at 6:39 AM, Linus Walleij [off-list ref] wrote:

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

quoted

This patch adds documentation for the gpio-switch binding. This binding
provides a mechanism to bind named links to gpio, with the primary
purpose of enabling standardised access to switches that might be standard
across a group of devices but implemented differently on each device.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>

As mentioned in the comment to the second patch, this solves the
following generic problem:

Expose a GPIO line to userspace using a specific name

That means basically naming GPIO lines and marking them as
"not used by the operating system".

This is something that has been proposed before, and postponed
because the kernel lacks the right infrastructure.

That doesn't necessarily mean we can't define a binding.

Markus Pargmann also did a series that add initial values to
hogs, which is the inverse usecase of this, where you want to
*output* something by default, then maybe also make it available
to userspace.

So what we need to see here is a patch series that does all of these
things:

- Name lines

- Sets them to initial values

- Mark them as read-only

- Mark them as "not used by the operating system" so that they
  can be default-exported to userspace.

No! This should not be a DT property.

Whether I want to control a GPIO in the kernel or userspace is not
known and can change over time. It could simply depend on kernel
config. There is also the case that a GPIO has no connection or kernel
driver until some time later when a DT overlay for an expansion board
is applied.

Rob

On Fri, Dec 11, 2015 at 3:06 PM, Rob Herring [off-list ref] wrote:

On Fri, Dec 11, 2015 at 6:39 AM, Linus Walleij [off-list ref] wrote:

quoted

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

quoted

This patch adds documentation for the gpio-switch binding. This binding
provides a mechanism to bind named links to gpio, with the primary
purpose of enabling standardised access to switches that might be standard
across a group of devices but implemented differently on each device.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>

As mentioned in the comment to the second patch, this solves the
following generic problem:

Expose a GPIO line to userspace using a specific name

That means basically naming GPIO lines and marking them as
"not used by the operating system".

This is something that has been proposed before, and postponed
because the kernel lacks the right infrastructure.

That doesn't necessarily mean we can't define a binding.

Yeah that's true. just saying that the other stuff was not merged
for this reason, but then we can have a look at Markus' bindings
in parallell, Markus can you repost them to this audience? Only
the bindings I mean.

quoted

Markus Pargmann also did a series that add initial values to
hogs, which is the inverse usecase of this, where you want to
*output* something by default, then maybe also make it available
to userspace.

So what we need to see here is a patch series that does all of these
things:

- Name lines

- Sets them to initial values

- Mark them as read-only

- Mark them as "not used by the operating system" so that they
  can be default-exported to userspace.

No! This should not be a DT property.

Whether I want to control a GPIO in the kernel or userspace is not
known and can change over time. It could simply depend on kernel
config. There is also the case that a GPIO has no connection or kernel
driver until some time later when a DT overlay for an expansion board
is applied.

That's correct. So from a DT point of view, what really matters is
to give things a name, and the hogs and initvals syntax already
has a structure for this that is in use
(from Documentation/devicetree/bindings/gpio/gpio.txt):

        qe_pio_a: gpio-controller at 1400 {
                compatible = "fsl,qe-pario-bank-a", "fsl,qe-pario-bank";
                reg = <0x1400 0x18>;
                gpio-controller;
                #gpio-cells = <2>;

                line_b {
                        gpio-hog;
                        gpios = <6 0>;
                        output-low;
                        line-name = "foo-bar-gpio";
                };
        };

Markus use this also for initial values. That could easily be used in
a budget version like this:

                line_b {
                        /* Just naming */
                        gpios = <6 0>;
                        line-name = "foo-bar-gpio";
                };

That could grow kind of big though. Or maybe not? How many
GPIO lines are actually properly named in a typical system?

The problem is that naming is usually imposed by consumers (they
are the ones who know how the line is used).

And then I do not mean naming it after the pin on the capsule
where it comes out as per the datasheet, but
naming it after the actual use.

Naming it after the hardware specs is something the operating
system can do, in Linux' case by the array char *names; inside the
struct gpio_chip and should not be part of the bindings IMO.

I would rather imagine this is something used in a top-level board
file naming it: "header-2-22" for the 22nd pin on some second header
on my BeagleBone Black or something like that. And those may not
be so vast in numbers so they could be named using this pattern.

Yours,
Linus Walleij

On Mon, Dec 14, 2015 at 8:28 AM, Linus Walleij [off-list ref] wrote:

On Fri, Dec 11, 2015 at 3:06 PM, Rob Herring [off-list ref] wrote:

quoted

On Fri, Dec 11, 2015 at 6:39 AM, Linus Walleij [off-list ref] wrote:

quoted

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

[...]

quoted

quoted

Markus Pargmann also did a series that add initial values to
hogs, which is the inverse usecase of this, where you want to
*output* something by default, then maybe also make it available
to userspace.

So what we need to see here is a patch series that does all of these
things:

- Name lines

- Sets them to initial values

- Mark them as read-only

- Mark them as "not used by the operating system" so that they
  can be default-exported to userspace.

No! This should not be a DT property.

Whether I want to control a GPIO in the kernel or userspace is not
known and can change over time. It could simply depend on kernel
config. There is also the case that a GPIO has no connection or kernel
driver until some time later when a DT overlay for an expansion board
is applied.

That's correct. So from a DT point of view, what really matters is
to give things a name, and the hogs and initvals syntax already
has a structure for this that is in use
(from Documentation/devicetree/bindings/gpio/gpio.txt):

        qe_pio_a: gpio-controller at 1400 {
                compatible = "fsl,qe-pario-bank-a", "fsl,qe-pario-bank";
                reg = <0x1400 0x18>;
                gpio-controller;
                #gpio-cells = <2>;

                line_b {
                        gpio-hog;
                        gpios = <6 0>;
                        output-low;
                        line-name = "foo-bar-gpio";
                };
        };

Markus use this also for initial values. That could easily be used in
a budget version like this:

                line_b {
                        /* Just naming */
                        gpios = <6 0>;
                        line-name = "foo-bar-gpio";
                };

That could grow kind of big though. Or maybe not? How many
GPIO lines are actually properly named in a typical system?

We should limit it to GPIOs with no connection to another node. For
example, I don't want to see a SD card detect in the list as that
should be in the SD host node. However, that is hard to enforce and
can change over time. Removing it would break userspace potentially.
Of course if the kernel starts own a signal that userspace used, then
that potentially breaks userspace regardless of the DT changing. OTOH,
using GPIOs in userspace is kind of use at your own risk.

The only real differences between this and Martyn's proposal are the
location of the nodes and having a compatible string. A compatible
string may be good to have. It indicates type of signal, but not
specific use. Similar to how gpio-key compatible defines the function,
but not what key code, or gpio-led does not say what color LED. A
compatible here could cover switches, mode/id/revision strapping
signals, jumpers, presence detect, etc.

The problem is that naming is usually imposed by consumers (they
are the ones who know how the line is used).

And then I do not mean naming it after the pin on the capsule
where it comes out as per the datasheet, but
naming it after the actual use.

Right. We need a way to query "I need the GPIO that does X" which
works across boards.

Naming it after the hardware specs is something the operating
system can do, in Linux' case by the array char *names; inside the
struct gpio_chip and should not be part of the bindings IMO.

Agreed. That just came up with STM32 gpio bindings[1].

I would rather imagine this is something used in a top-level board
file naming it: "header-2-22" for the 22nd pin on some second header
on my BeagleBone Black or something like that. And those may not
be so vast in numbers so they could be named using this pattern.

Exactly. This is one of the cases I care about. However, this is not
really a function, but it is SOC independent at least.

We also have to consider how to handle add-on boards. We probably need
a connector node which can remap connector signals to host signals in
order to have overlays independent of the host board DT. For GPIOs
this is probably a gpio-map property similar to interrupt-map. The
complicated part will be connectors that require pinmux setup of the
host.

Rob

[1] https://lkml.org/lkml/2015/12/11/600

Hi,

On Monday 14 December 2015 09:45:48 Rob Herring wrote:

On Mon, Dec 14, 2015 at 8:28 AM, Linus Walleij [off-list ref] wrote:

quoted

On Fri, Dec 11, 2015 at 3:06 PM, Rob Herring [off-list ref] wrote:

quoted

On Fri, Dec 11, 2015 at 6:39 AM, Linus Walleij [off-list ref] wrote:

quoted

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

[...]

quoted

quoted

quoted

Markus Pargmann also did a series that add initial values to
hogs, which is the inverse usecase of this, where you want to
*output* something by default, then maybe also make it available
to userspace.

So what we need to see here is a patch series that does all of these
things:

- Name lines

- Sets them to initial values

- Mark them as read-only

- Mark them as "not used by the operating system" so that they
  can be default-exported to userspace.

No! This should not be a DT property.

Whether I want to control a GPIO in the kernel or userspace is not
known and can change over time. It could simply depend on kernel
config. There is also the case that a GPIO has no connection or kernel
driver until some time later when a DT overlay for an expansion board
is applied.

That's correct. So from a DT point of view, what really matters is
to give things a name, and the hogs and initvals syntax already
has a structure for this that is in use
(from Documentation/devicetree/bindings/gpio/gpio.txt):

        qe_pio_a: gpio-controller at 1400 {
                compatible = "fsl,qe-pario-bank-a", "fsl,qe-pario-bank";
                reg = <0x1400 0x18>;
                gpio-controller;
                #gpio-cells = <2>;

                line_b {
                        gpio-hog;
                        gpios = <6 0>;
                        output-low;
                        line-name = "foo-bar-gpio";
                };
        };

Markus use this also for initial values. That could easily be used in
a budget version like this:

                line_b {
                        /* Just naming */
                        gpios = <6 0>;
                        line-name = "foo-bar-gpio";
                };

That could grow kind of big though. Or maybe not? How many
GPIO lines are actually properly named in a typical system?

We should limit it to GPIOs with no connection to another node. For
example, I don't want to see a SD card detect in the list as that
should be in the SD host node. However, that is hard to enforce and
can change over time. Removing it would break userspace potentially.
Of course if the kernel starts own a signal that userspace used, then
that potentially breaks userspace regardless of the DT changing. OTOH,
using GPIOs in userspace is kind of use at your own risk.

I see this a bit differently. I would really like to see the each GPIO having
two different names:
- GPIO label: This is the name of the GPIO line in the schematic for example
- GPIO use (this is the current semantic of the GPIO name): The use of a GPIO,
  e.g. 'sd-card-detect', 'LED', ...

I think it would be good to describe all GPIO labels in gpiochip subnodes as
gpio-hogging introduced it. This would offer a use-independent naming. The use
of the function could be defined in the device node that is using this gpio.

As an example perhaps something like this:

	&gpiochip {
		some_interrupt {
			gpios = <4 0>;
			line-name = "some_interrupt_line";
		};

		line_b {
			gpios = <6 0>;
			line-name = "line-b";
		};
	};

	randomswitch {
		compatible = "gpio-switch";
		gpios = <&gpiochip 4 0>;
		use = "action-trigger";
		read-only;
	};

Also this does seem kind of inconsistent with gpio-hogging and the proposed
gpio-initval. gpio-hogging is defined in subnodes of the gpio chip while
gpio-switches are not. As "gpio-switch" is not really any kind of device it
would perhaps make sense to keep this consistent with gpio-hogging as well and
define it in the same subnodes?
I would be happy about any consistent way.

The only real differences between this and Martyn's proposal are the
location of the nodes and having a compatible string. A compatible
string may be good to have. It indicates type of signal, but not
specific use. Similar to how gpio-key compatible defines the function,
but not what key code, or gpio-led does not say what color LED. A
compatible here could cover switches, mode/id/revision strapping
signals, jumpers, presence detect, etc.

quoted

The problem is that naming is usually imposed by consumers (they
are the ones who know how the line is used).

And then I do not mean naming it after the pin on the capsule
where it comes out as per the datasheet, but
naming it after the actual use.

Right. We need a way to query "I need the GPIO that does X" which
works across boards.

quoted

Naming it after the hardware specs is something the operating
system can do, in Linux' case by the array char *names; inside the
struct gpio_chip and should not be part of the bindings IMO.

Agreed. That just came up with STM32 gpio bindings[1].

quoted

I would rather imagine this is something used in a top-level board
file naming it: "header-2-22" for the 22nd pin on some second header
on my BeagleBone Black or something like that. And those may not
be so vast in numbers so they could be named using this pattern.

Exactly. This is one of the cases I care about. However, this is not
really a function, but it is SOC independent at least.

We also have to consider how to handle add-on boards. We probably need
a connector node which can remap connector signals to host signals in
order to have overlays independent of the host board DT. For GPIOs
this is probably a gpio-map property similar to interrupt-map. The
complicated part will be connectors that require pinmux setup of the
host.

Also what about hotplugging gpio-chips? Is there any mechanism to let the
'gpio-switch' know that the GPIO is not there anymore?

Best Regards,

Markus

-- 
Pengutronix e.K.                           |                             |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |
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On 11/12/15 09:08, Linus Walleij wrote:

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

quoted

Select Chromebooks have gpio attached to switches used to cause the
firmware to enter alternative modes of operation and/or control other
device characteristics (such as write protection on flash devices). This
patch adds a driver that exposes a read-only interface to allow these
signals to be read from user space.

This functionality has been generalised to provide support for any device
with device tree support which needs to identify a gpio as being used for a
specific task.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>

If you want to do this thing, also propose a device tree binding document
for "gpio-switch".

But first (from Documentation/gpio/drivers-on-gpio.txt):

- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
   can generate interrupts in response to a key press. Also supports debounce.

This one generates input events from gpio. I'm not looking to generate 
events.

- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
   GPIO line cannot generate interrupts, so it needs to be periodically polled
   by a timer.

Ditto (but using a polled mechanism rather than interrupts)

- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
   external connector status, such as a headset line for an audio driver or an
   HDMI connector. It will provide a better userspace sysfs interface than GPIO.

This appears to be exclusively for monitoring insertion events, or am I 
missing something?

So you mean none of these apply for this case?

No, I'm looking for a mechanism to identify GPIO as connected to a 
specific signal, which is provided across multiple devices, but which 
might be implemented subtly differently on different platforms (i.e. 
active high/low) and on different GPIO lines.

Second: what you want to do is export a number of GPIOs with certain names
to userspace. This is something very generic and should be implemented
as such, not as something Chromebook-specific.

I'd agree that my first implementation was ChromeBook specific, but I'm 
fairly sure that my last attempt wasn't. I've mentioned ChromeBooks as 
an example of an existing use case.

Patches like that has however already been suggested, and I have NACKed
them because the GPIO sysfs ABI is insane, and that is why I am refactoring
the world to create a proper chardev ABI for GPIO instead. See:
http://marc.info/?l=linux-gpio&m=144550276512673&w=2

I can certainly understand the rationale for the changes that you are 
proposing, though do worry that it does make it a bit tougher to use 
from scripting languages. I see that the question of how to provide 
functionality equivalent to the above was raised and no answer was 
forthcoming. Is there a plan for supporting the identification of a GPIO 
line serving a specific purpose?

What is the status of the mentioned patch series?

Martyn

So for the moment, NACK on this, please participate in creating the
*right* ABI for GPIO instead of trying to shoehorn stuff into the dying
sysfs ABI.

Yours,
Linus Walleij

On Wed, Dec 16, 2015 at 11:11 AM, Martyn Welch
[off-list ref] wrote:

quoted

Patches like that has however already been suggested, and I have NACKed
them because the GPIO sysfs ABI is insane, and that is why I am
refactoring
the world to create a proper chardev ABI for GPIO instead. See:
http://marc.info/?l=linux-gpio&m=144550276512673&w=2

I can certainly understand the rationale for the changes that you are
proposing, though do worry that it does make it a bit tougher to use from
scripting languages.

The idea is to provide commandline tools in the kernel tools/gpio subdir
to satisfy the needs of scripting. "lsgpio" today is just one example,
nothing stops us from having a tool called just "gpio-sak"
(GPIO swiss army knife) that will be tailored for scripting.

I see that the question of how to provide functionality
equivalent to the above was raised and no answer was forthcoming. Is there a
plan for supporting the identification of a GPIO line serving a specific
purpose?

Yes by name.

What is the status of the mentioned patch series?

They stubled into a few dozen architecture issues in the GPIO
subsystem so I am busy refactoring the whole know universe :D

But I still intend to persue the series.

Yours,
Linus Walleij

Reviving this thread...

On Tue, Dec 15, 2015 at 3:09 AM, Markus Pargmann [off-list ref] wrote:

Hi,

On Monday 14 December 2015 09:45:48 Rob Herring wrote:

quoted

On Mon, Dec 14, 2015 at 8:28 AM, Linus Walleij [off-list ref] wrote:

quoted

On Fri, Dec 11, 2015 at 3:06 PM, Rob Herring [off-list ref] wrote:

quoted

On Fri, Dec 11, 2015 at 6:39 AM, Linus Walleij [off-list ref] wrote:

quoted

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

[...]

quoted

quoted

quoted

Markus Pargmann also did a series that add initial values to
hogs, which is the inverse usecase of this, where you want to
*output* something by default, then maybe also make it available
to userspace.

So what we need to see here is a patch series that does all of these
things:

- Name lines

- Sets them to initial values

- Mark them as read-only

- Mark them as "not used by the operating system" so that they
  can be default-exported to userspace.

No! This should not be a DT property.

Whether I want to control a GPIO in the kernel or userspace is not
known and can change over time. It could simply depend on kernel
config. There is also the case that a GPIO has no connection or kernel
driver until some time later when a DT overlay for an expansion board
is applied.

That's correct. So from a DT point of view, what really matters is
to give things a name, and the hogs and initvals syntax already
has a structure for this that is in use
(from Documentation/devicetree/bindings/gpio/gpio.txt):

        qe_pio_a: gpio-controller at 1400 {
                compatible = "fsl,qe-pario-bank-a", "fsl,qe-pario-bank";
                reg = <0x1400 0x18>;
                gpio-controller;
                #gpio-cells = <2>;

                line_b {
                        gpio-hog;
                        gpios = <6 0>;
                        output-low;
                        line-name = "foo-bar-gpio";
                };
        };

Markus use this also for initial values. That could easily be used in
a budget version like this:

                line_b {
                        /* Just naming */
                        gpios = <6 0>;
                        line-name = "foo-bar-gpio";
                };

That could grow kind of big though. Or maybe not? How many
GPIO lines are actually properly named in a typical system?

We should limit it to GPIOs with no connection to another node. For
example, I don't want to see a SD card detect in the list as that
should be in the SD host node. However, that is hard to enforce and
can change over time. Removing it would break userspace potentially.
Of course if the kernel starts own a signal that userspace used, then
that potentially breaks userspace regardless of the DT changing. OTOH,
using GPIOs in userspace is kind of use at your own risk.

I see this a bit differently. I would really like to see the each GPIO having
two different names:

I think we are saying the same thing...

- GPIO label: This is the name of the GPIO line in the schematic for example

Yes.

- GPIO use (this is the current semantic of the GPIO name): The use of a GPIO,
  e.g. 'sd-card-detect', 'LED', ...

This should be determined from the compatible string and/or -gpios
prefix. This is the what the function is and "label" is which one.

I think it would be good to describe all GPIO labels in gpiochip subnodes as
gpio-hogging introduced it. This would offer a use-independent naming. The use
of the function could be defined in the device node that is using this gpio.

I think I agree here. You may have a defined function without any
connection to another node. I also think we should encourage simple
GPIO bindings like gpio-leds to be child nodes. Having them at the
top-level is kind of arbitrary. Of course, allowing for both is
required.

As an example perhaps something like this:

        &gpiochip {
                some_interrupt {
                        gpios = <4 0>;
                        line-name = "some_interrupt_line";
                };

                line_b {
                        gpios = <6 0>;
                        line-name = "line-b";
                };
        };

        randomswitch {
                compatible = "gpio-switch";
                gpios = <&gpiochip 4 0>;
                use = "action-trigger";
                read-only;
        };

Also this does seem kind of inconsistent with gpio-hogging and the proposed
gpio-initval. gpio-hogging is defined in subnodes of the gpio chip while
gpio-switches are not. As "gpio-switch" is not really any kind of device it
would perhaps make sense to keep this consistent with gpio-hogging as well and
define it in the same subnodes?
I would be happy about any consistent way.

Yes, as well as gpio leds, keys, etc. bindings. The key is you would
need to be able to start with something minimal and extend it with
specific compatibles.

[...]

quoted

We also have to consider how to handle add-on boards. We probably need
a connector node which can remap connector signals to host signals in
order to have overlays independent of the host board DT. For GPIOs
this is probably a gpio-map property similar to interrupt-map. The
complicated part will be connectors that require pinmux setup of the
host.

Also what about hotplugging gpio-chips? Is there any mechanism to let the
'gpio-switch' know that the GPIO is not there anymore?

There are certainly issues around hotplug and GPIOs. If the
gpio-switch device is a child of the gpio controller, then it should
be possible.

Rob

On Wednesday 02 March 2016 10:03:27 Rob Herring wrote:

Reviving this thread...

On Tue, Dec 15, 2015 at 3:09 AM, Markus Pargmann [off-list ref] wrote:

quoted

Hi,

On Monday 14 December 2015 09:45:48 Rob Herring wrote:

quoted

On Mon, Dec 14, 2015 at 8:28 AM, Linus Walleij [off-list ref] wrote:

quoted

On Fri, Dec 11, 2015 at 3:06 PM, Rob Herring [off-list ref] wrote:

quoted

On Fri, Dec 11, 2015 at 6:39 AM, Linus Walleij [off-list ref] wrote:

quoted

On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
[off-list ref] wrote:

[...]

quoted

quoted

quoted

Markus Pargmann also did a series that add initial values to
hogs, which is the inverse usecase of this, where you want to
*output* something by default, then maybe also make it available
to userspace.

So what we need to see here is a patch series that does all of these
things:

- Name lines

- Sets them to initial values

- Mark them as read-only

- Mark them as "not used by the operating system" so that they
  can be default-exported to userspace.

No! This should not be a DT property.

Whether I want to control a GPIO in the kernel or userspace is not
known and can change over time. It could simply depend on kernel
config. There is also the case that a GPIO has no connection or kernel
driver until some time later when a DT overlay for an expansion board
is applied.

That's correct. So from a DT point of view, what really matters is
to give things a name, and the hogs and initvals syntax already
has a structure for this that is in use
(from Documentation/devicetree/bindings/gpio/gpio.txt):

        qe_pio_a: gpio-controller at 1400 {
                compatible = "fsl,qe-pario-bank-a", "fsl,qe-pario-bank";
                reg = <0x1400 0x18>;
                gpio-controller;
                #gpio-cells = <2>;

                line_b {
                        gpio-hog;
                        gpios = <6 0>;
                        output-low;
                        line-name = "foo-bar-gpio";
                };
        };

Markus use this also for initial values. That could easily be used in
a budget version like this:

                line_b {
                        /* Just naming */
                        gpios = <6 0>;
                        line-name = "foo-bar-gpio";
                };

That could grow kind of big though. Or maybe not? How many
GPIO lines are actually properly named in a typical system?

We should limit it to GPIOs with no connection to another node. For
example, I don't want to see a SD card detect in the list as that
should be in the SD host node. However, that is hard to enforce and
can change over time. Removing it would break userspace potentially.
Of course if the kernel starts own a signal that userspace used, then
that potentially breaks userspace regardless of the DT changing. OTOH,
using GPIOs in userspace is kind of use at your own risk.

I see this a bit differently. I would really like to see the each GPIO having
two different names:

I think we are saying the same thing...

quoted

- GPIO label: This is the name of the GPIO line in the schematic for example

Yes.

quoted

- GPIO use (this is the current semantic of the GPIO name): The use of a GPIO,
  e.g. 'sd-card-detect', 'LED', ...

This should be determined from the compatible string and/or -gpios
prefix. This is the what the function is and "label" is which one.

quoted

I think it would be good to describe all GPIO labels in gpiochip subnodes as
gpio-hogging introduced it. This would offer a use-independent naming. The use
of the function could be defined in the device node that is using this gpio.

I think I agree here. You may have a defined function without any
connection to another node. I also think we should encourage simple
GPIO bindings like gpio-leds to be child nodes. Having them at the
top-level is kind of arbitrary. Of course, allowing for both is
required.

quoted

As an example perhaps something like this:

        &gpiochip {
                some_interrupt {
                        gpios = <4 0>;
                        line-name = "some_interrupt_line";
                };

                line_b {
                        gpios = <6 0>;
                        line-name = "line-b";
                };
        };

        randomswitch {
                compatible = "gpio-switch";
                gpios = <&gpiochip 4 0>;
                use = "action-trigger";
                read-only;
        };

Also this does seem kind of inconsistent with gpio-hogging and the proposed
gpio-initval. gpio-hogging is defined in subnodes of the gpio chip while
gpio-switches are not. As "gpio-switch" is not really any kind of device it
would perhaps make sense to keep this consistent with gpio-hogging as well and
define it in the same subnodes?
I would be happy about any consistent way.

Yes, as well as gpio leds, keys, etc. bindings. The key is you would
need to be able to start with something minimal and extend it with
specific compatibles.

So if I understand you right it would be better to handle most
distinctions using compatibles? Something like this?

	&gpiochip {
		some_led {
			compatible = "gpio-leds";
			default-state = "on";
			gpios = <3 0>;
			line-name = "leda";
		};

		some_switch {
			compatible = "gpio-switch", "gpio-initval";
			gpios = <4 0>;
			line-name = "switch1";

			/* This is used by gpio-initval in case gpio-switch is not implemented */
			output-low;
		};

		some_interrupt {
			gpios = <5 0>;
			line-name = "some_interrupt_line";
		};

		line_b {
			gpios = <6 0>;
			line-name = "line-b";
		};
	};

	randomcomponent {
		gpios = <&gpiochip 5 0>;
		read-only;
	};

In this case there is a gpio-led embedded within the gpiochip node. Also
'some_switch' is a gpio-switch and falls back to being gpio-initval.
This way we can replace it at some point with a real driver. But if it
doesn't exist we can at least initialize the GPIO properly.

As you suggested this would open up the possibilities for all
compatibles and drivers for single gpios.

Best Regards,

Markus

[...]

quoted

quoted

We also have to consider how to handle add-on boards. We probably need
a connector node which can remap connector signals to host signals in
order to have overlays independent of the host board DT. For GPIOs
this is probably a gpio-map property similar to interrupt-map. The
complicated part will be connectors that require pinmux setup of the
host.

Also what about hotplugging gpio-chips? Is there any mechanism to let the
'gpio-switch' know that the GPIO is not there anymore?

There are certainly issues around hotplug and GPIOs. If the
gpio-switch device is a child of the gpio controller, then it should
be possible.

Rob

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