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

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Date:   Thu, 18 Nov 2021 18:59:23 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
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 05:37:02PM +0100, Bartosz Golaszewski wrote:
> 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 awkward so in this iteration it's been replaced by a system that is more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®.

Fine with me then!

Thanks for doing this all, I know it's a bit delayed in terms of getting
into upstream.

Btw, gpio-mockup testing scripts have an issue that the number of lines to
check overflow is hardcoded and since x86_64 switched to 1024 from 512 it
reveals the issue. Does gpio-sim solve this in a better way (like telling
to user space the ngpios, etc)?


-- 
With Best Regards,
Andy Shevchenko