Re: [PATCH 3/3] sample: rust: Add GPIO consumer sample driver

[Kohei Ito <koheiito.dev@gmail.com>]

Hi Bartosz, Alexandre,

Thanks both for the feedback.

On Mon, Sep 14, 2026 at 04:36:48AM -0400, Bartosz Golaszewski wrote:
> On Mon, 14 Sep 2026 03:41:29 +0200, Alexandre Courbot
> <acourbot@nvidia.com> said:
> > On Sun Sep 13, 2026 at 5:46 PM JST, Kohei Ito wrote:
> >> Hi, Bartosz,
> >>
> >> On Thu, Sep 10, 2026 at 12:37:17AM -0700, Bartosz Golaszewski wrote:
> >>> On Sun, 6 Sep 2026 10:45:51 +0200, Kohei Ito <koheiito.dev@gmail.com> said:
> >>> > Add a sample driver to demonstrate the use of the Rust GPIO APIs.
> >>> >
> >>> > Signed-off-by: Kohei Ito <koheiito.dev@gmail.com>
> >>> > ---
> >>>
> >>> I don't like samples as they rarely get built or tested. We seem to already
> >>> have kunit support for rust, wouldn't it make more sense to implement a kunit
> >>> module for rust GPIO abstractions? If we don't have provider abstractions, you
> >>> should be able to reuse gpio-sim as the GPIO controller for testing just by
> >>> instantiating simulated GPIO devices.
> >>
> >> Thank you for your suggestion.
> >>
> >> I assume the kunit module you have in mind would be implemented like
> >> `gpiolib-kunit.c`. Based on your comment, I agree that a kunit-based
> >> approach is more appropriate than a sample driver.
> >>
> >> However, as far as I know, we don't yet have Rust abstractions for
> >> platform_device registration and software_node, which are required to
> >> implement a kunit-based module for testing GPIO consumer APIs. Given the
> >> current state of Rust for Linux, I think creating a sample driver is a
> >> more practical approach for now. I would like to consider migrating to a
> >> kunit-based module as future work.
> >
> > The problem is that this sample driver never probes, so in effect it is
> > only ever compile-tested. Without support for the provider API, you need
> > to include a small C fixture providing a GPIO chip for it to be actually
> > runtime-tested.
> >
> > Doing the same using KUnit would involve building the `gpio_chip` using a
> > bunch of unsafe statements working with the C bindings (which would then
> > in turn require `gpio/driver.h` to be added), so I guess we'll want to
> > wait until we have a proper Rust provider API to go that direction.
> >
> 
> No, I was thinking about the gpio-sim module which is implemented as a platform
> driver which you can describe with a software node and then register to create
> a simulated GPIO provider against which the consumer APIs in rust could be
> tested.
> 
> To that end, we'd need to just register a platform device from rust and AFAICT,
> there are already APIs for that, except for the software nodes.
> 
> Bart

I looked into this further, but couldn't find any Rust abstraction for
registering a new `platform_device` (no file contains a platform device
registration function such as `platform_device_register()`,
`platform_device_register_*()`, or `platform_device_alloc()`/
`platform_device_add()`). So, we can't write a fully Rust kunit module
for the GPIO consumer API, at least for v7.3-rc3.

The best approach I think is a combination of a Rust consumer driver and
a C kunit module dedicated to testing the consumer driver. This is
almost the same as `drivers/gpio/gpiolib-kunit.c`, except that instead
of directly exercising the GPIO consumer APIs, the module I'm suggesting
exercises them indirectly, through the Rust consumer driver.

Best regards,
Kohei Ito