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

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Message-ID: <CACRpkdbwx+6xB0=rwm60=2jM4OfyDKxkwAEZMgU=10LuijsW1A@mail.gmail.com>
Date:   Thu, 14 Oct 2021 18:51:30 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Shreeya Patel <shreeya.patel@...labora.com>
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>
Subject: Re: [PATCH] gpio: Return EPROBE_DEFER if gc->to_irq is NULL

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.

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?

Yours,
Linus Walleij