Re: gpio-sch GPIO_SYSFS access

[Darren Hart <dvhart@linux.intel.com>]

On 02/07/2013 02:09 AM, Linus Walleij wrote:
> On Thu, Feb 7, 2013 at 1:58 AM, Darren Hart <dvhart@linux.intel.com> wrote:
> 
>> Is it that some other driver has claimed these GPIO lines? If so, how do
>> I determine which one?
> 
> Yes I think that could be it, the driver would need to call
> gpio_export() for it to also be accessible in sysfs.
> 
> Configure in debugfs and check the file "gpio" in debugfs
> to figure out the client.
> 
> Yours,
> Linus Walleij
> 

I found gpio_export() as you suggested above and instrumented it. What I
found was that it was not getting called at all. As I understand it,
calling gpiochip_export() should make the gpiochip# appear in
/sys/class/gpio and then I should be able to configure which lines are
exported via the /sys/class/gpio/export file.

I haven't yet found how gpio-pch differs from gpio-sch that causes the
gpio-pch chip to appear in sysfs and the gpio-sch one not to. I did
patch gpio-sch with a request and export loop:

$ git diff drivers/gpio/gpio-sch.c
diff --git a/drivers/gpio/gpio-sch.c b/drivers/gpio/gpio-sch.c
index 8cadf4d..79783c1 100644
--- a/drivers/gpio/gpio-sch.c
+++ b/drivers/gpio/gpio-sch.c
@@ -188,7 +188,7 @@ static struct gpio_chip sch_gpio_resume = {
 static int __devinit sch_gpio_probe(struct platform_device *pdev)
 {
        struct resource *res;
-       int err, id;
+       int err, id, gpio;

        id = pdev->id;
        if (!id)
@@ -243,10 +243,24 @@ static int __devinit sch_gpio_probe(struct
platform_device *p
        if (err < 0)
                goto err_sch_gpio_core;

+       /* DEBUG: export all the core GPIOS */
+       for (gpio = sch_gpio_core.base;
+            gpio < sch_gpio_core.base + sch_gpio_core.ngpio; gpio++) {
+               gpio_request(gpio, "gpio-sch");
+               gpio_export(gpio, true);
+       }
+
        err = gpiochip_add(&sch_gpio_resume);
        if (err < 0)
                goto err_sch_gpio_resume;

+       /* DEBUG: export all the resume GPIOS */
+       for (gpio = sch_gpio_resume.base;
+            gpio < sch_gpio_resume.base + sch_gpio_resume.ngpio; gpio++) {
+               gpio_request(gpio, "gpio-sch");
+               gpio_export(gpio, true);
+       }
+
        return 0;

 err_sch_gpio_resume:


With this both the gpiochip# and gpio# entries appear in sysfs. However,
unlike those for the gpio-pch lines, these report an error in the sysfs
interface:

/sys/class/gpio# ls *
ls: gpio0: No such file or directory

ls: gpio1: No such file or directory

ls: gpio10: No such file or directory

ls: gpio11: No such file or directory

ls: gpio12: No such file or directory

ls: gpio13: No such file or directory

ls: gpio2: No such file or directory

ls: gpio3: No such file or directory

ls: gpio4: No such file or directory

ls: gpio5: No such file or directory

ls: gpio6: No such file or directory

ls: gpio7: No such file or directory

ls: gpio8: No such file or directory

ls: gpio9: No such file or directory

ls: gpiochip0: No such file or directory

ls: gpiochip5: No such file or directory

export    unexport



gpiochip244:

base       label      ngpio      power      subsystem  uevent

Clearly I'm still missing something. I've read through gpio.txt a couple
times and each time piece a bit more together. I'll do that again, but I
still suspect I'm missing something fundamental here. In particular, I
still don't understand how the gpio-pch and gpio-sch drivers are create
such different results.


Ultimately what I'm looking to do is configure a new board such that 8
of the gpio-sch lines are configured as buttons and LEDs which are
physically attached to the board (4 of each). I was hoping to use this
interface to understand the ins and outs (haha) of the GPIO subsystem.
Ultimately I believe I need either a "platform-device" or possibly an
ACPI DSDT from the firmware to properly describe the GPIO lines and
their direction. Ultimately, these should be driven by the gpio-keys and
gpio-led drivers - at least that's my current understanding.

Any help clarifying some of this would be greatly appreciated.

Thanks for the time!

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel