linux-kernel - Re: [PATCH v2 2/4] gpio - Add EXAR XRA1403 SPI GPIO expander driver

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Date:   Wed, 26 Apr 2017 16:42:18 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Benjamin Henrion <zoobab@...il.com>
Cc:     Nandor Han <nandor.han@...com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Alexandre Courbot <gnurou@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Semi Malinen <semi.malinen@...com>
Subject: Re: [PATCH v2 2/4] gpio - Add EXAR XRA1403 SPI GPIO expander driver

On Tue, Apr 25, 2017 at 9:07 AM, Benjamin Henrion <zoobab@...il.com> wrote:

> I doubt you will be able to convince the majority of people toggling
> GPIOs via a simple shell script to switch to write a complex C
> program. Not to mention cross compilation and the libraries
> dependencies here.

I do not need to convince anyone, I'm not into politics.

The way to attract users to the character device is by offering better
features... so we smack in the following goodies:

- Need a userspace ABI? No more need to select CONFIG_GPIO_SYSFS!
  The chardev is always there for any gpio chip in newer kernels!
  Board vendors would have to actively delete core code to disable it!

- Need open drain? The chardev will support that, the sysfs will never.

- Need to set/get multiple lines with a single context switch? Chardev
  does this. Also the set operation will turn into a single register write
  if your driver implements .set_multiple()

- All future needs: line biasing? Schmitt triggers? Drive strengths?
  All that will use the character device, and the sysfs ABI will never
  support any of it.

> Is there some good cli tools to access the new char device? If they
> are shipped with most distros, that would reduce the pain.

I have those that come with the kernel:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/gpio

Then libgpiod as mentioned:
https://github.com/brgl/libgpiod/tree/master/src/tools

Yours,
Linus Walleij