linux-kernel - Re: [PATCH v9 0/4] gpio-sim: configfs-based GPIO simulator

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Date:   Thu, 18 Nov 2021 17:37:02 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Kent Gibson <warthog618@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Shuah Khan <shuah@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v9 0/4] gpio-sim: configfs-based GPIO simulator

On Thu, Nov 18, 2021 at 4:50 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Thu, Nov 18, 2021 at 03:51:38PM +0100, Bartosz Golaszewski wrote:
> > This is another shot at the gpio-sim testing module. As there was no
> > reasoning with configfs maintainers for many months, this time the whole
> > concept of committable items has been dropped. Instead, each configfs
> > chip item (or rather a group - more on that later) exposes a new
> > attribute called 'live'. Writing 1 to it brings the chip on-line
> > (registers the platform device) and writing 0 tears it down.
> >
> > There are some caveats to that approach - for example: we can't block
> > the user-space from deleting chip items when chips are live but is just
> > handled by silently destroying the chip device in the background.
> >
> > Andy (rightfully) pointed out that parsing of the lists of line names is
> > awkward so in this iteration it's been replaced by a system that is more
> > elegant and will allow to easily extend configuration options for
> > specific GPIO lines. This is achieved by turning the chip's configfs
> > item into a configfs group and allowing the user-space to create
> > additional items inside it. The items must be called line<offset> (e.g.
> > line0, line12 etc.) where the offset part indicates to the module the
> > offset for which given item stores the configuration for. Within each
> > such line item, there are additional attributes that allow specifying
> > configuration for specific lines. Currently we only support the 'name'
> > attribute but I plan to extend that to support GPIO hogging too.
>
> One question here. Since you know how the driver looks like in both cases
> (with and without committable items), would it be possible to modify what
> you proposed here to the former one in case ConfigFS gains the feature?
>

This would completely change the user interface unfortunately. We
could extend it but we would need to keep this one too most likely.

TBH I don't see the committable items merged anytime soon, and this is
GoodEnough®.

Bart