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

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Date:   Thu, 23 Mar 2017 13:34:37 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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

On Thu, Mar 23, 2017 at 01:03:56PM +0100, Geert Uytterhoeven wrote:
> Hi Uwe,
> 
> 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?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |