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From:   Bartosz Golaszewski <brgl@bgdev.pl>
Date:   Tue, 8 Sep 2020 19:03:30 +0200
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Subject: Re: [PATCH 23/23] Documentation: gpio: add documentation for gpio-mockup
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Kent Gibson <warthog618@gmail.com>,
        linux-gpio <linux-gpio@vger.kernel.org>,
        linux-doc <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>
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On Mon, Sep 7, 2020 at 2:22 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Mon, Sep 07, 2020 at 02:06:15PM +0200, Bartosz Golaszewski wrote:
> > On Mon, Sep 7, 2020 at 1:53 PM Andy Shevchenko
> > <andriy.shevchenko@linux.intel.com> wrote:
> > >
> > > On Mon, Sep 07, 2020 at 12:26:34PM +0200, Bartosz Golaszewski wrote:
> > > > On Mon, Sep 7, 2020 at 11:59 AM Andy Shevchenko
> > > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > >
> > > > > On Fri, Sep 04, 2020 at 08:15:59PM -0700, Randy Dunlap wrote:
> > > > > > On 9/4/20 8:45 AM, Bartosz Golaszewski wrote:
> > > > >
> > > > > ...
> > > > >
> > > > > > > +GPIO Testing Driver
> > > > > > > +===================
> > > > > > > +
> > > > > > > +The GPIO Testing Driver (gpio-mockup) provides a way to create simulated GPIO
> > > > > > > +chips for testing purposes. There are two ways of configuring the chips exposed
> > > > > > > +by the module. The lines can be accessed using the standard GPIO character
> > > > > > > +device interface as well as manipulated using the dedicated debugfs directory
> > > > > > > +structure.
> > > > > >
> > > > > > Could configfs be used for this instead of debugfs?
> > > > > > debugfs is ad hoc.
> > > > >
> > > > > Actually sounds like a good idea.
> > > > >
> > > >
> > > > Well, then we can go on and write an entirely new mockup driver
> > > > (ditching module params and dropping any backwards compatibility)
> > > > because we're already using debugfs for line values.
> > > >
> > > > How would we pass the device properties to configfs created GPIO chips
> > > > anyway? Devices seem to only be created using mkdir. Am I missing
> > > > something?
> > >
> > > Same way how USB composite works, no?
> > >
> >
> > OK, so create a new chip directory in configfs, configure it using
> > some defined configfs attributes and then finally instantiate it from
> > sysfs?
> >
> > Makes sense and is probably the right way to go. Now the question is:
> > is it fine to just entirely remove the previous gpio-mockup? Should we
> > keep some backwards compatibility? Should we introduce an entirely new
> > module and have a transition period before removing previous
> > gpio-mockup?
> >
> > Also: this is a testing module so to me debugfs is just fine. Is
> > configfs considered stable ABI like sysfs?
>
> Yes it is.  Or at least until you fix all existing users so that if you
> do change it, no one notices it happening :)
>

Got it. One more question: the current debugfs interface we're using
in gpio-mockup exists to allow to read current values of GPIO lines in
output mode (check how the user drives dummy lines) and to set their
simulated pull-up/pull-down resistors (what values the user reads in
input mode).

This works like this: in /sys/kernel/debug/gpio-mockup every dummy
chip creates its own directory (e.g.
/sys/kernel/debug/gpio-mockup/gpiochip0) and inside this directory
there's an attribute per line named after the line's offset (e.g.
/sys/kernel/debug/gpio-mockup/gpiochip0/4). Writing 0 or 1 to this
attribute sets the pull resistor. Reading from it yields the current
value (0 or 1 as well).

This is pretty non-standard so I proposed to put it in debugfs. If we
were to use configfs - is this where something like this should go? Or
rather sysfs? Is it even suitable/acceptable for sysfs?

Thanks,
Bartosz