linux-kernel - Re: [PATCH] gpio: Return EPROBE_DEFER if gc->to

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Date:   Tue, 19 Oct 2021 03:44:48 +0530
From:   Shreeya Patel <shreeya.patel@...labora.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Wolfram Sang <wsa@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-i2c <linux-i2c@...r.kernel.org>, kernel@...labora.com
Subject: Re: [PATCH] gpio: Return EPROBE_DEFER if gc->to_irq is NULL


On 14/10/21 10:21 pm, Linus Walleij wrote:
> On Thu, Oct 14, 2021 at 1:05 PM Shreeya Patel
> <shreeya.patel@...labora.com> wrote:
>
>> We are racing the registering of .to_irq when probing the
>> i2c driver. This results in random failure of touchscreen
>> devices.
>>
>> Following errors could be seen in dmesg logs when gc->to_irq is NULL
>>
>> [2.101857] i2c_hid i2c-FTS3528:00: HID over i2c has not been provided an Int IRQ
>> [2.101953] i2c_hid: probe of i2c-FTS3528:00 failed with error -22
>>
>> To avoid this situation, defer probing until to_irq is registered.
>>
>> This issue has been reported many times in past and people have been
>> using workarounds like changing the pinctrl_amd to built-in instead
>> of loading it as a module or by adding a softdep for pinctrl_amd into
>> the config file.
>>
>> References :-
>> https://bugzilla.kernel.org/show_bug.cgi?id=209413
>> https://github.com/Syniurge/i2c-amd-mp2/issues/3
>>
>> Signed-off-by: Shreeya Patel <shreeya.patel@...labora.com>
> I understand the issue.
>
> There is one problem.
>
>> @@ -3084,7 +3084,7 @@ int gpiod_to_irq(const struct gpio_desc *desc)
>>
>>                  return retirq;
>>          }
>> -       return -ENXIO;
>> +       return -EPROBE_DEFER;
> If you after five minutes plug in a USB FTDI or similar UART thing
> with a GPIO expander, and someone request an IRQ from
> one of those lines (they do not support interrupts), why should
> it return -EPROBE_DEFER?
>
> The point is that I think this will in certain circumstances return
> a bogus error.

I was worried about the same but didn't really know under what scenario 
this could occur.
Thanks for pointing this out.

>
> We cannot merge this other than with a fat comment above:
>
> /*
>   * This is semantically WRONG because the -EPROBE_DEFER
>   * is really just applicable during system bring-up.
>   */
> return -EPROBE_DEFER;
>
> Can we use some kind of late_initcall() to just switch this over
> to -ENXIO after a while?


I have sent a v2 which tries to fix this in an easy way. Let me know 
what do you
think about that approach or else we could also think about using 
late_initcall().



Thanks,
Shreeya Patel


> Yours,
> Linus Walleij