linux-kernel - Re: [PATCH v2 00/10] Device / Driver and PCI Rust abstractions

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Date: Wed, 19 Jun 2024 14:36:21 +0200
From: Danilo Krummrich <dakr@...hat.com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, bhelgaas@...gle.com,
	ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me, a.hindborg@...sung.com,
	aliceryhl@...gle.com, airlied@...il.com, fujita.tomonori@...il.com,
	lina@...hilina.net, pstanner@...hat.com, ajanulgu@...hat.com,
	lyude@...hat.com, robh@...nel.org, daniel.almeida@...labora.com,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org,
	Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2 00/10] Device / Driver and PCI Rust abstractions

On Wed, Jun 19, 2024 at 05:34:07PM +0530, Viresh Kumar wrote:
> On 19-06-24, 01:39, Danilo Krummrich wrote:
> > - move base device ID abstractions to a separate source file (Greg)
> > - remove `DeviceRemoval` trait in favor of using a `Devres` callback to
> >   unregister drivers
> > - remove `device::Data`, we don't need this abstraction anymore now that we
> >   `Devres` to revoke resources and registrations
> 
> Hi Danilo,
> 
> I am working on writing bindings for CPUFreq drivers [1] and was
> looking to rebase over staging/rust-device, and I am not sure how to
> proceed after device::Data is dropped now.
> 
> What I was doing at probe() was something like this:
> 
>     fn probe(dev: &mut platform::Device, id_info: Option<&Self::IdInfo>) -> Result<Self::Data> {
>         let data = Arc::<DeviceData>::from(kernel::new_device_data!(
>             cpufreq::Registration::new(),
>             (),
>             "CPUFreqDT::Registration"
>         )?);
> 
>         ...
> 
>         // Need a mutable object to be passed to register() here.
>         data.registrations()
>             .ok_or(ENXIO)?
>             .as_pinned_mut()
>             .register(c_str!("cpufreq-dt"), ...)?;
> 
>         Ok(data)
>     }
> 
> The register() function of cpufreq core needs a mutable pointer to
> `self` and it worked earlier as Data used a RevocableMutex. But with
> Devres, we don't have a Mutex anymore and devres.try_access() doesn't
> give a mutable object.

If you want to split `cpufreq::Registration` in `new()` and `register()`, you
probably want to pass the registration object to `Devres` in `register()`
instead.

However, I wouldn't recommend splitting it up (unless you absolutely have to),
it's way cleaner (and probably less racy) if things are registered once the
registration is created.

> 
> I am a bit confused on how to get this going. I looked at how PCI bus
> is implemented in the staging/dev but I couldn't find an end driver
> using this work.

The PCI abstraction did not need to change for that, since it uses the
generalized `driver::Registration`, which is handled by the `Module` structure
instead.

However, staging/dev also contains the `drm::drv::Registration` type [1], which
in principle does the same thing as `cpufreq::Registration` just for a DRM
device.

If you're looking for an example driver making use of this, please have a look
at Nova [1].

[1] https://github.com/Rust-for-Linux/linux/blob/staging/dev/rust/kernel/drm/drv.rs
[2] https://gitlab.freedesktop.org/drm/nova/-/blob/nova-next/drivers/gpu/drm/nova/driver.rs

> 
> Maybe I am making an mistake and missing the obvious.
> 
> Thanks.
> 
> -- 
> viresh
> 
> [1] https://lore.kernel.org/all/cover.1717750631.git.viresh.kumar@linaro.org/
>