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

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Thu, Nov 18, 2021 at 5:59 PM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxx> 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 awkward so in this iteration it's been replaced by a system that is more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)?
>

Yeah the selftests need fixing now.

No, there's no fix for that in gpio-sim - probe() will just fail.
Which makes me think - maybe we should synchronously wait when writing
to 'live' for the probe to return (for instance setup a notifier) so
that we know if the chip probed correctly. Then we can notify the
user-space about the error destroy the device too.

Bart