linux-kernel - Re: [PATCH 1/8] gpiolib: check the return value of gpio_chip::get

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Message-ID: <7c045d13-146f-448c-ad73-60069059b242@samsung.com>
Date: Wed, 19 Feb 2025 10:13:58 +0100
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: Linus Walleij <linus.walleij@...aro.org>, linux-gpio@...r.kernel.org,
	linux-kernel@...r.kernel.org, Bartosz Golaszewski
	<bartosz.golaszewski@...aro.org>, stable@...r.kernel.org
Subject: Re: [PATCH 1/8] gpiolib: check the return value of
 gpio_chip::get_direction()

On 19.02.2025 09:50, Bartosz Golaszewski wrote:
> On Wed, Feb 19, 2025 at 9:38 AM Marek Szyprowski
> <m.szyprowski@...sung.com> wrote:
>> On 10.02.2025 11:51, Bartosz Golaszewski wrote:
>>> From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
>>>
>>> As per the API contract - gpio_chip::get_direction() may fail and return
>>> a negative error number. However, we treat it as if it always returned 0
>>> or 1. Check the return value of the callback and propagate the error
>>> number up the stack.
>>>
>> This change breaks bcm2835 pincontrol/gpio driver (and probably others)
>> in next-20250218. The problem is that some gpio lines are initially
>> configured as alternate function (i.e. uart) and .get_direction returns
>> -EINVAL for them, what in turn causes the whole gpio chip fail to
>> register. Here is the log with WARN_ON() added to line
>> drivers/pinctrl/bcm/pinctrl-bcm2835.c:350 from Raspberry Pi 4B:
>>
>> Any suggestions how to fix this issue? Should we add
>> GPIO_LINE_DIRECTION_UNKNOWN?
>>
> That would be quite an intrusive change and not something for the
> middle of the release cycle. I think we need to revert to the previous
> behavior for this particular use-case: check ret for EINVAL and assume
> it means input as it's the "safe" setting. Now the question is - can
> this only happen during the chip registration or should we filter out
> EINVAL at each gpiod_get_direction() call?

IMHO it will be enough to use that workaround only in the 
gpiochip_add_data_with_key() function. The other functions modified by 
the $subject patch are strictly related to input or output gpio mode of 
operation, so having the line set to proper input/output state seems to 
be justified.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland