[PATCH v3 0/3] am335x: set pinmux registers from pins debug file

The patch was born from the need to change the slew rate of the LCD pins
of a custom AM335x board during EMC tests. The AM335x, as described in a
note in section 9.1 of its reference manual [1], is unable to write
pinmux registers from user space. The series now makes it possible to
write these registers from the pins debug file.

[1] https://www.ti.com/lit/ug/spruh73q/spruh73q.pdf


Changes in v3:
- Use strncpy_from_user() instead of copy_from_user().
- Do not shadow the error code returned by kstrtouint().
- Change pin_dbg_set() interface (char *buf --> unsigned int val).
- Describe pin_dbg_set().
- Remove CONFIG_DEV_MEM dependency.
- Change pcs_pin_dbg_set() interface (char *buf -> unsigned int val).

Changes in v2:
- Remove CONFIG_SOC_AM33XX dependency.

Dario Binacchi (3):
  docs/pinctrl: update `pins' description under debugfs
  pinctrl: core: configure pinmux from pins debug file
  pinctrl: single: set pinmux from pins debug file

 Documentation/driver-api/pin-control.rst |  3 ++
 drivers/pinctrl/core.c                   | 63 +++++++++++++++++++++++-
 drivers/pinctrl/pinctrl-single.c         | 13 +++++
 include/linux/pinctrl/pinctrl.h          |  4 ++
 4 files changed, 81 insertions(+), 2 deletions(-)

-- 
2.17.1


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As described in section 9.1 of the TI reference manual for AM335x [1],
"For writing to the control module registers, the MPU will need to be in
privileged mode of operation and writes will not work from user mode".
By adding the pin_dbg_set helper to pcs_pinctrl_ops it will be possible
to write these registers from the pins debug:

cd /sys/kernel/debug/pinctrl/44e10800.pinmux-pinctrl-single/
echo <pin-number> <reg-value> >pins

[1] https://www.ti.com/lit/ug/spruh73q/spruh73q.pdf

Signed-off-by: Dario Binacchi <redacted>

---

Changes in v3:
- Remove CONFIG_DEV_MEM dependency.
- Change pcs_pin_dbg_set() interface (char *buf -> unsigned int val).

Changes in v2:
- Remove CONFIG_SOC_AM33XX dependency.

 drivers/pinctrl/pinctrl-single.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/pinctrl/pinctrl-single.c b/drivers/pinctrl/pinctrl-single.c
index 2c9c9835f375..1b75236563cf 100644
--- a/drivers/pinctrl/pinctrl-single.c
+++ b/drivers/pinctrl/pinctrl-single.c

@@ -313,6 +313,18 @@ static void pcs_pin_dbg_show(struct pinctrl_dev *pctldev,
 	seq_printf(s, "%zx %08x %s ", pa, val, DRIVER_NAME);
 }
 
+static int pcs_pin_dbg_set(struct pinctrl_dev *pctldev, unsigned int pin,
+			   unsigned int val)
+{
+	struct pcs_device *pcs;
+	unsigned int mux_bytes;
+
+	pcs = pinctrl_dev_get_drvdata(pctldev);
+	mux_bytes = pcs->width / BITS_PER_BYTE;
+	pcs->write(val, pcs->base + pin * mux_bytes);
+	return 0;
+}
+
 static void pcs_dt_free_map(struct pinctrl_dev *pctldev,
 				struct pinctrl_map *map, unsigned num_maps)
 {

@@ -331,6 +343,7 @@ static const struct pinctrl_ops pcs_pinctrl_ops = {
 	.get_group_name = pinctrl_generic_get_group_name,
 	.get_group_pins = pinctrl_generic_get_group_pins,
 	.pin_dbg_show = pcs_pin_dbg_show,
+	.pin_dbg_set = pcs_pin_dbg_set,
 	.dt_node_to_map = pcs_dt_node_to_map,
 	.dt_free_map = pcs_dt_free_map,
 };

-- 
2.17.1


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Hello Dario,

On 5/20/21 11:27 PM, Dario Binacchi wrote:

As described in section 9.1 of the TI reference manual for AM335x [1],
"For writing to the control module registers, the MPU will need to be in
privileged mode of operation and writes will not work from user mode".
By adding the pin_dbg_set helper to pcs_pinctrl_ops it will be possible
to write these registers from the pins debug:

this makes little sense, because it should be possible to write to the
registers without introduction of this interface, pin group/function
is the right level of abstraction for your particular task.

cd /sys/kernel/debug/pinctrl/44e10800.pinmux-pinctrl-single/
echo <pin-number> <reg-value> >pins

[1] https://www.ti.com/lit/ug/spruh73q/spruh73q.pdf

Signed-off-by: Dario Binacchi <redacted>

I strongly object against this new interface to be added.

--
Best wishes,
Vladimir

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On Thu, May 20, 2021 at 10:27 PM Dario Binacchi [off-list ref] wrote:

The patch was born from the need to change the slew rate of the LCD pins
of a custom AM335x board during EMC tests. The AM335x, as described in a
note in section 9.1 of its reference manual [1], is unable to write
pinmux registers from user space. The series now makes it possible to
write these registers from the pins debug file.

I am going to hold this series back pending more discussion on the ABI
and whether it is useful.

Yours,
Linus Walleij

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