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

[Bartosz Golaszewski]

On Mon, 21 Sep 2026 15:12:01 +0200, Kohei Ito <koheiito.dev@xxxxxxxxx> said:
> 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@xxxxxxxxxx> 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@xxxxxxxxx> said:
>> >>> > Add a sample driver to demonstrate the use of the Rust GPIO APIs.
>> >>> >
>> >>> > Signed-off-by: Kohei Ito <koheiito.dev@xxxxxxxxx>
>> >>> > ---
>> >>>
>> >>> 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.
>

Right, I must have misremembered this.

> 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.
>

Can't you just call a C function registering the platform device from unsafe
rust?

Bart