Re: [PATCH] [RFC] gpio: Retry deferred GPIO hogging on pin range change

From: Geert Uytterhoeven
Date: Sun Jun 21 2015 - 04:53:16 EST

Hi Alex,

On Sun, Jun 21, 2015 at 9:21 AM, Alexandre Courbot <gnurou@xxxxxxxxx> wrote:
> On Tue, Jun 16, 2015 at 9:36 PM, Geert Uytterhoeven
> <geert+renesas@xxxxxxxxx> wrote:
>> If a GPIO driver uses gpiochip_add_pin_range() (which is usually the
>> case for GPIO/PFC combos), the GPIO hogging mechanism configured from DT
>> doesn't work:
>> requesting hog GPIO lcd0 (chip r8a7740_pfc, offset 176) failed
>> The actual error code is -517 == -EPROBE_DEFER.
>> The problem is that PFC+GPIO registration is handled in multiple steps:
>> 1. pinctrl_register(),
>> 2. gpiochip_add(),
>> 3. gpiochip_add_pin_range().
>> Configuration of the hogs is handled in gpiochip_add():
>> gpiochip_add
>> of_gpiochip_add
>> of_gpiochip_scan_hogs
>> gpiod_hog
>> gpiochip_request_own_desc
>> __gpiod_request
>> chip->request
>> pinctrl_request_gpio
>> pinctrl_get_device_gpio_range
>> However, at this point the GPIO controller hasn't been added to
>> pinctrldev_list yet, so the range can't be found, and the operation fails
>> with -EPROBE_DEFER.
>> - Exchanging the order of the calls to gpiochip_add() and
>> gpiochip_add_pin_range() is not an option, as the latter depends on
>> initialization done by the former.
>> - Just moving the call of of_gpiochip_scan_hogs() from gpiochip_add()
>> to gpiochip_add_pin_range() is also not an option, as the latter is
>> optional, and thus not used by all drivers.
>> Hence if of_gpiochip_scan_hogs() fails with -EPROBE_DEFER, call it
>> again every time the pin range is changed, until it succeeded.
>> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
>> ---
>> Questions:
>> - Is there a better solution to handle this?
> I do not understand the issue well enough to propose a better
> solution, but I really hope there is one. This turns GPIO into a
> slightly bigger yarn mess that what it already is and does not help
> understanding how probing is performed. Yielding in the middle of
> adding hogs and re-trying later sounds like a recipe for issues like
> hogs being added several times.
> So I am not really fond of this, to be honest. The GPIO hogging
> mechanism is still quite new, so there is certainly a way to fix such
> issues by addressing the fundamental cause instead of duct taping it?

Sure, I'm all for fixing this properly, hence the "RFC" and my questions.

I also don't understand how this interacts with non-PFC drivers calling
- gpio-em, but only for legacy platform devices, which are no longer used
(I will remove the legacy support),
- gpio-rcar, but only for legacy platform devices, which is used on R-Car
Gen1 only until -legacy is removed,
- gpiolib-of, which handles this for the bulk of modern GPIO drivers using
the "gpio-ranges" and "gpio-ranges-group-names" properties in DT.

When I noticed the failure on r8a7740/armadillo (sh-pfc provides both pfc
and gpio), I tried GPIO hogging on r8a7791/koelsch (sh-pfc provides pfc
only, gpio-rcar provides gpio, "gpio-ranges" is in DT), and there it worked
fine without my patch.


>> - Should the pin ranges be configured by passing an array of data to
>> gpiochip_add() instead of having calls to gpiochip_add_pin_range()?
>> That would require changing all drivers.
>> - What happens if you have multiple hogs in multiple ranges?
>> The first hog(s) may be configured multiple times. Is that a problem?



Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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