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

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Message-ID: <CAMuHMdXiz5hqW=fm-2H1aNKq_gXD2o0508jD2XjxYUvKoEvHdw@mail.gmail.com>
Date:   Sat, 4 Mar 2017 16:35:46 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     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>
Subject: Re: [PATCH 4/4] tty/serial: sh-sci: remove uneeded IS_ERR_OR_NULL calls

Hi Uwe,

On Fri, Mar 3, 2017 at 8:44 PM, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
> On Fri, Mar 03, 2017 at 08:21:05PM +0100, Geert Uytterhoeven wrote:
>> > diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
>> > index 91e7dddbf72c..2f4cdd4e7b4f 100644
>> > --- a/drivers/tty/serial/sh-sci.c
>> > +++ b/drivers/tty/serial/sh-sci.c
>> > @@ -3022,7 +3022,7 @@ static int sci_probe_single(struct platform_device *dev,
>> >                 return ret;
>> >
>> >         sciport->gpios = mctrl_gpio_init(&sciport->port, 0);
>> > -       if (IS_ERR(sciport->gpios) && PTR_ERR(sciport->gpios) != -ENOSYS)
>> > +       if (IS_ERR(sciport->gpios))
>> >                 return PTR_ERR(sciport->gpios);
>>
>> Now the sh-sci driver fails to probe on legacy platforms where GPIOLIB=n.
>> The check for -ENOSYS made it succeed before.
>
> That's right, intended and the only option that's save (for some
> definition of save; the obvious downside is that there is no
> /dev/tty$whatever for you).

That's not just a downside, but a plain regression on legacy platforms that
do not use GPIO flow control.

> Ignoring -ENOSYS is only ok if your device doesn't have a cts-gpio. If
> it has, ignoring -ENOSYS hides bugs because the driver sends data while
> it shouldn't or cannot signal the other side that it should stop (or
> start) a transmission.

Mctrl_gpio supports modern platforms with GPIOLIB and DT or ACPI only.
On legacy platforms, you cannot use GPIO flow control (except when using a
custom implementation, which is out-of-scope here), so the issue of silently
running without cts-gpio on these platforms is moot.

> So the options are:
>  - enable GPIOLIB (maybe enforce that by letting the driver select it)
>  - introduce a CONFIG_HALFGPIOLIB that makes gpiod_get_optional and
>    mctrl_gpio_init return only then -ENOSYS if there is a gpio specified
>    and NULL otherwise.
>
>> > Then mctrl_gpio_to_gpiod isn't called. I don't have a machine to test
>> > this, but I think currently this makes the machine barf to continue
>> > here because with sciport->gpios = ERR_PTR(-ENOSYS) calling
>> >
>> >         mctrl_gpio_to_gpiod(sciport->gpios, ...)
>> >
>> > is a bad idea.
>>
>> If sciport->gpios == ERR_PTR(-ENOSYS), CONFIG_GPIOLIB is not enabled, the
>> feature is not available, and mctrl_gpio_to_gpiod() will not
>> dereference the error
>> pointer.
>
> Ah, makes sense.
>
>> >> Perhaps mctrl_gpio_to_gpiod() should always return NULL if !CONFIG_GPIOLIB?
>> >
>> > No, mctrl_gpio_to_gpiod is right. You are only supposed to call it if
>> > mctrl_gpio_init succeeded.
>>
>> Then I have to add checks for sciport->gpios == ERR_PTR(-ENOSYS)...
>
> No, ignoring -ENOSYS is wrong.

How else to handle this in a driver that supports both modern (DT && GPIOLIB)
and legacy platforms?

sh-sci is not only used on DT-only architectures like arm, arm64, and h8300,
but also on SuperH, where some platforms use GPIOLIB, others don't, and none
of them use DT yet (jcore doesn't matter, as it doesn't use sh-sci).

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