Hi,

On Tue, 5 Jul 2016 20:04:47 +0200 Uwe Kleine-K?nig wrote:

On Tue, Jul 05, 2016 at 09:05:46AM -0500, Rob Herring wrote:

quoted

On Fri, Jul 01, 2016 at 08:42:13AM +0200, Uwe Kleine-K?nig wrote:

quoted

This uses the same approach that is already used for spi, i2c and
several other controllers to ensure a consistent numbering independent
of probe order. This is in use for several gpio drivers that already now
use of_alias_get_id(np, "gpio").

Like SPI and I2C, I'm against further abuse of aliases for this purpose 
[1].

I considered spi and i2c the good examples here :-|

+1

quoted

quoted

Signed-off-by: Uwe Kleine-K?nig <redacted>
---
Hello,

Linus requested such a patch as part of a change that introduces
this mechanism to the gpio-omap driver[1]. IMHO this is better done in a
separate patch, so here it comes.

Best regards
Uwe

[1] http://thread.gmane.org/gmane.linux.kernel.gpio/17399/focus=17629

 Documentation/devicetree/bindings/gpio/gpio.txt | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/Documentation/devicetree/bindings/gpio/gpio.txt b/Documentation/devicetree/bindings/gpio/gpio.txt
index 68d28f62a6f4..5dbacc8f094a 100644
--- a/Documentation/devicetree/bindings/gpio/gpio.txt
+++ b/Documentation/devicetree/bindings/gpio/gpio.txt

@@ -227,6 +227,24 @@ Example of two SOC GPIO banks defined as gpio-controller nodes:
 		#gpio-cells = <2>;
 	};
 
+Usually the GPIO banks in SoCs are ordered, that is there is a dedicated "first
+gpio bank". To fix this ordering in the device tree use aliases starting at 0
+(even if the first bank is called "GPIO1" in the hardware reference).
+This is necessary/handy to ensure deterministical numbering of GPIOs and GPIO
+controllers.

Why is deterministic numbering needed?

in my case (with a pre 4.8 kernel) it's to control GPIO48 with
/sys/class/gpio/gpio48. But also when using the gpio chardev device
(that will hit 4.8-rc1 AFAIK) there is one device file per gpio chip.

Now consider a user who wants to control/debug direction and value of
GPIO48 (or GPIO2.16 for the chardev case). The strait forward approach
is to use /sys/class/gpio/gpio48 (or /dev/dontknowthename2 with offset
16). I doubt there is a platform where it didn't work like this up to
now and I'd consider it a userspace breakage to force the user to know
that the 2nd gpio bank is located at address 0x53fd0000 and so to lookup
the gpio bank below /sys/bus/platform/devices/53fd0000.gpio/.

On an i.MX25 device I currently see:

root at hostname:/sys/bus/gpio/devices ls -l
lrwxrwxrwx    1 root     root             0 Jul  5 20:52 gpiochip0 -> ../../../devices/platform/soc/53f00000.aips/53f9c000.gpio/gpiochip0
lrwxrwxrwx    1 root     root             0 Jul  5 20:52 gpiochip1 -> ../../../devices/platform/soc/53f00000.aips/53fa4000.gpio/gpiochip1
lrwxrwxrwx    1 root     root             0 Jul  5 20:52 gpiochip2 -> ../../../devices/platform/soc/53f00000.aips/53fcc000.gpio/gpiochip2
lrwxrwxrwx    1 root     root             0 Jul  5 20:52 gpiochip3 -> ../../../devices/platform/soc/53f00000.aips/53fd0000.gpio/gpiochip3

That is we have:

	Hardware name | software gpiochip
	    GPIO4     |      gpiochip0
	    GPIO3     |      gpiochip1
	    GPIO1     |      gpiochip2
	    GPIO2     |	     gpiochip3

I bet that's the probe order because when sorted by address (and so
by order in the device tree) we have exactly this ordering. (Compare
with $(grep gpio@ arch/arm/boot/dts/imx25.dtsi).)

For a new interface this is OK, still I predict users will complain if
the numbers used don't match naturally the hardware names. And IMHO they
are right.

IMO the burden of doing such lookups should be put on the computer, not
the human who is using it. That's what computers are good at.
If aliases in DT are the wrong way to a straightforward numbering
scheme for use by humans, we should introduce a different way to achieve
this.


Lothar Wa?mann