linux-kernel - Re: [PATCH v3 1/2] rust: irq: add support for request

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Message-ID: <aD3LHfkmuXR-wPzm@google.com>
Date: Mon, 2 Jun 2025 16:02:37 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Daniel Almeida <daniel.almeida@...labora.com>, 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>, 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 1/2] rust: irq: add support for request_irq()

On Thu, May 15, 2025 at 03:52:29PM +0200, Danilo Krummrich wrote:
> On Thu, May 15, 2025 at 03:45:28PM +0200, Danilo Krummrich wrote:
> > On Thu, May 15, 2025 at 10:16:33AM -0300, Daniel Almeida wrote:
> > > 
> > > >> 
> > > >> Well, not really, because this impl PinInit can be assigned to something larger
> > > >> that is already pinned, like drm::Device::Data for example, which is (or was)
> > > >> already behind an Arc, or any other private data in other subsystems.
> > > >> 
> > > >> IIUC what you proposed has yet another indirection. If we reuse the example
> > > >> from above, that would be an Arc for the drm Data, and another Arc for the
> > > >> handler itself?
> > > > 
> > > > Can't you implement Handler for drm::Device::Data and e.g. make Registration
> > > > take an Arc<T: Handler>?
> > > 
> > > No, because drivers may need more than one handler. Panthor needs 3, for
> > > example, for 3 different lines.
> > > 
> > > > 
> > > > The irq::Registration itself doesn't need to be allocated dynamically, so it'd
> > > > still be a single allocation, no?
> > > > 
> > > 
> > > Right, the registrations don't, but the handlers do.
> > 
> > Why does the handler need to be allocated dynamically?
> > 
> > What about something like the following?
> > 
> > 	pub struct Registration<T, H: Handler<T>> { ... };
> > 
> > 	pub trait Handler<T> {
> > 	   fn handle_irq(&T) -> IrqReturn;
> > 	}
> > 
> > 	// Could be `drm::Device::Data`.
> > 	struct MyData { ... };
> > 
> > 	// Implements `Handler<MyData>`.
> > 	struct IRQHandler1;
> > 	struct IRQHandler2;
> > 
> > 	// `data` is `Arc<MyData>`
> > 	irq::Registration::<IRQHandler1>::new(data, ...);
> > 	irq::Registration::<IRQHandler2>::new(data, ...);
> > 
> > With that you can have as many IRQs as you want without any additional
> > allocation.
> 
> Alternatively we could also do the following, which is probably simpler.
> 
> 	pub struct Registration<H: Handler> { ... };
> 
> 	pub trait Handler {
> 	   fn handle_irq(&self) -> IrqReturn;
> 	}
> 
> 	// Could be `drm::Device::Data`.
> 	struct MyData { ... };
> 
> 	// Implements `Handler`.
> 	struct IRQHandler1(Arc<MyData>);
> 	struct IRQHandler2(Arc<MyData>);
> 
> 	// `data` is `Arc<MyData>`
> 	let handler1 = IRQHandler1::new(data);
> 	let handler2 = IRQHandler2::new(data);
> 
> 	irq::Registration::new(handler1, ...);
> 	irq::Registration::new(handler2, ...);

Yeah there's no reason to *force* the user to store an Arc. The user
could store data that is only accessed by the irq callback without an
Arc. I agree that for shared content you probably want to access it
through an Arc.

Alice