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

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Date:   Thu, 23 Mar 2017 12:11:06 +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

Hello,

On Thu, Mar 23, 2017 at 11:20:39AM +0100, Geert Uytterhoeven wrote:
> But having the error breaks setups where the GPIO is optional and does
> not exist.

so the right way forward is to check harder in the situation where
-ENOSYS was returned before to determine if there is really no GPIO to
be used. "Oh, there are hints that there is no GPIO (GPIOLIB=n), so lets
assume there isn't." is wrong.

Can we please properly fix the problem instead of papering over it?

> 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.

> > So you exchanged many obvious and easy to fix problems with a few hard
> > ones. I don't agree that's a good idea, but you seem to be willing to
> > try it. Good luck.
> 
> Yeah, before drivers had to explicitly ignore -ENOSYS if they want to
> support platforms with and without GPIOLIB. Bad...

Doing things right is sometimes not maximally easy. But that is no
excuse to do it wrong.

Best regards
Uwe

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