linux-kernel - Re: [PATCH v5 09/12] gpio: aggregator: handle runtime registration of gpio_desc in gpiochip

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Message-ID: <c3b9c494-599e-4d99-8645-589c1c0c106c@bootlin.com>
Date: Wed, 7 May 2025 12:10:44 +0200
From: Thomas Richard <thomas.richard@...tlin.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: Linus Walleij <linus.walleij@...aro.org>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Bartosz Golaszewski <brgl@...ev.pl>,
 Geert Uytterhoeven <geert+renesas@...der.be>, Kees Cook <kees@...nel.org>,
 Andy Shevchenko <andy@...nel.org>, linux-gpio@...r.kernel.org,
 linux-kernel@...r.kernel.org, thomas.petazzoni@...tlin.com,
 DanieleCleri@...on.eu, GaryWang@...on.com.tw, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v5 09/12] gpio: aggregator: handle runtime registration of
 gpio_desc in gpiochip_fwd

Hi Andy,

Thanks again for the review.

On 5/7/25 08:34, Andy Shevchenko wrote:
> On Tue, May 6, 2025 at 6:21 PM Thomas Richard
> <thomas.richard@...tlin.com> wrote:
>>
>> Add request() callback to check if the GPIO descriptor was well registered
>> in the gpiochip_fwd before using it. This is done to handle the case where
>> GPIO descriptor is added at runtime in the forwarder.
>>
>> If at least one GPIO descriptor was not added before the forwarder
>> registration, we assume the forwarder can sleep as if a GPIO is added at
>> runtime it may sleep.
> 
> ...
> 
>>  {
>>         struct gpiochip_fwd *fwd = gpiochip_get_data(chip);
>>
>> +       /*
>> +        * get_direction() is called during gpiochip registration, return input
>> +        * direction if there is no descriptor for the line.
>> +        */
>> +       if (!test_bit(offset, fwd->valid_mask))
>> +               return GPIO_LINE_DIRECTION_IN;
> 
> Can you remind me why we choose a valid return for invalid line? From
> a pure code perspective this should return an error.

I reproduced gpiolib behavior. During gpiochip registration, we get the
direction of all lines. In the case the line is not valid, it is marked
as input if direction_input operation exists, otherwise it is marked as
output. [1]

But in fact we could return an error and the core will mark the line as
input. Maybe ENODEV ?

[1]
https://elixir.bootlin.com/linux/v6.15-rc5/source/drivers/gpio/gpiolib.c#L1105-L1123

Regards,

Thomas