linux-kernel - Re: [PATCH v3 2/2] rust: platform: add irq accessors

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Message-ID: <aCsK7s0qepzIiA-l@pollux>
Date: Mon, 19 May 2025 12:41:50 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Daniel Almeida <daniel.almeida@...labora.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 2/2] rust: platform: add irq accessors

On Wed, May 14, 2025 at 04:20:52PM -0300, Daniel Almeida wrote:
> These accessors can be used to retrieve an IRQ from a platform device by
> index or name. The IRQ can then be used to request the line using
> irq::request::Registration::register() and
> irq::request::ThreadedRegistration::register().
> 
> Signed-off-by: Daniel Almeida <daniel.almeida@...labora.com>
> ---
>  rust/kernel/platform.rs | 52 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
> index 4917cb34e2fe8027d3d861e90de51de85f006735..1acaebf38d99d06f93fa13b0b356671ea77ed97a 100644
> --- a/rust/kernel/platform.rs
> +++ b/rust/kernel/platform.rs
> @@ -188,6 +188,58 @@ impl Device {
>      fn as_raw(&self) -> *mut bindings::platform_device {
>          self.0.get()
>      }
> +
> +    /// Returns an IRQ for the device by index.
> +    pub fn irq_by_index(&self, index: u32) -> Result<u32> {
> +        // SAFETY: `self.as_raw` returns a valid pointer to a `struct platform_device`.
> +        let res = unsafe { bindings::platform_get_irq(self.as_raw(), index) };
> +
> +        if res < 0 {
> +            return Err(Error::from_errno(res));
> +        }
> +
> +        Ok(res as u32)
> +    }
> +
> +    /// Same as [`Self::irq_by_index`] but does not print an error message if an IRQ
> +    /// cannot be obtained.
> +    pub fn optional_irq_by_index(&self, index: u32) -> Result<u32> {
> +        // SAFETY: `self.as_raw` returns a valid pointer to a `struct platform_device`.
> +        let res = unsafe { bindings::platform_get_irq_optional(self.as_raw(), index) };
> +
> +        if res < 0 {
> +            return Err(Error::from_errno(res));
> +        }
> +
> +        Ok(res as u32)
> +    }
> +
> +    /// Returns an IRQ for the device by name.
> +    pub fn irq_by_name(&self, name: &CStr) -> Result<u32> {
> +        // SAFETY: `self.as_raw` returns a valid pointer to a `struct platform_device`.
> +        let res = unsafe { bindings::platform_get_irq_byname(self.as_raw(), name.as_char_ptr()) };
> +
> +        if res < 0 {
> +            return Err(Error::from_errno(res));
> +        }
> +
> +        Ok(res as u32)
> +    }
> +
> +    /// Same as [`Self::irq_by_name`] but does not print an error message if an IRQ
> +    /// cannot be obtained.
> +    pub fn optional_irq_by_name(&self, name: &CStr) -> Result<u32> {
> +        // SAFETY: `self.as_raw` returns a valid pointer to a `struct platform_device`.
> +        let res = unsafe {
> +            bindings::platform_get_irq_byname_optional(self.as_raw(), name.as_char_ptr())
> +        };
> +
> +        if res < 0 {
> +            return Err(Error::from_errno(res));
> +        }
> +
> +        Ok(res as u32)
> +    }

I don't like the indirection of claiming a u32 representing the IRQ number from
a bus device and stuffing it into an irq::Registration.

It would be better we we'd make it impossible (or at least harder) for a driver
to pass the wrong number to irq::Registration.

I see two options:

  1) Make the platform::Device accessors themselves return an
     irq::Registration.

  2) Make the platform::Device accessors return some kind of transparent cookie,
     that drivers can't create themselves that can be fed into the
     irq::Registration.

My preference would be 1) if there's no major ergonomic issue with that.