linux-kernel - Re: [PATCH 4/4] tty/serial: sh-sci: remove uneeded IS_ERR_OR

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Date:   Thu, 23 Mar 2017 13:44:19 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Richard Genoud <richard.genoud@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Janusz Uzycki <j.uzycki@...roma.com.pl>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH 4/4] tty/serial: sh-sci: remove uneeded IS_ERR_OR_NULL calls

Hi Uwe,

On Thu, Mar 23, 2017 at 1:34 PM, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
> On Thu, Mar 23, 2017 at 01:03:56PM +0100, Geert Uytterhoeven wrote:
>> On Thu, Mar 23, 2017 at 12:11 PM, Uwe Kleine-König
>> <u.kleine-koenig@...gutronix.de> wrote:
>> >> Make sure to enable all drivers and subsystems you need when building
>> >> your kernel. That's always true. And may indeed be hard to debug (e.g. what
>> >> kernel options do I need to make systemd work?).
>> >
>> > It's worse here. If you forget to enable a driver the device isn't bound
>> > and that's obvious to diagnose. When ignoring an optional GPIO there
>> > might be a device that claims to work but fails to do so. (e.g. you
>> > write to memory, write() returns 0, but the data never landed there.)
>> >
>> >> > write(2) and close(2) succeed most of the time, too. Still it's not a
>> >> > good idea to not check the return value. Or let the kernel return
>> >> > success unconditionally.
>> >>
>> >> Writing all bytes passed in the buffer is "optional" in another sense than
>> >> an "optional" GPIO: you must retry the write, while you can continue if
>> >> an optional GPIO is not present.
>> >
>> > And that is the point. You can continue *iff* the optional GPIO is not
>> > present. The patch in question removes the ability to determine if that
>> > GPIO is present and claims it is not present.
>>
>> If you forget to enable a driver/subsystem, you sometimes cannot determine
>> if the device is present or not neither.
>>
>> Hence it boils down to "knowing" if there is a GPIO or not.
>> So, when can there be a GPIO?
>>   1. The GPIO is described in DT.
>>      => Not an issue, as DT GPIO implies GPIOLIB,
>>   2. The GPIO is described in legacy platform data.
>>      => The platform code should make sure GPIOLIB is selected when needed.
>>
>> Issue solved?
>
> I like it better to not rely on platform code to do the right thing.

;-)

> Maybe we can make gpiod_get_optional look like this:
>
>         if (!dev->of_node && isnt_a_acpi_device(dev) && !IS_ENABLED(GPIOLIB))
>                 return NULL;
>         else
>                 return -ENOSYS;
>
> I don't know how isnt_a_acpi_device looks like, probably it involves
> CONFIG_ACPI and/or dev->acpi_node.
>
> This should be safe and still comfortable for legacy platforms, isn't it?

Yes, that should do the trick.

No feedback from me about ACPI.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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