linux-kernel - Re: [PATCH v6 3/6] rust: irq: add support for non-threaded IRQs and handlers

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Message-ID: <aG0F2nFpjrW1HmKR@google.com>
Date: Tue, 8 Jul 2025 11:49:46 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <lossin@...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>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	"Krzysztof Wilczyński" <kwilczynski@...nel.org>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v6 3/6] rust: irq: add support for non-threaded IRQs and handlers

On Mon, Jul 07, 2025 at 10:30:27PM +0200, Benno Lossin wrote:
> On Mon Jul 7, 2025 at 6:18 PM CEST, Daniel Almeida wrote:
> > Alice,
> >
> > […]
> >
> >>> +/// The value that can be returned from an IrqHandler or a ThreadedIrqHandler.
> >>> +pub enum IrqReturn {
> >>> +    /// The interrupt was not from this device or was not handled.
> >>> +    None,
> >>> +
> >>> +    /// The interrupt was handled by this device.
> >>> +    Handled,
> >>> +}
> >>> +
> >>> +impl IrqReturn {
> >>> +    fn into_inner(self) -> u32 {
> >>> +        match self {
> >>> +            IrqReturn::None => bindings::irqreturn_IRQ_NONE,
> >>> +            IrqReturn::Handled => bindings::irqreturn_IRQ_HANDLED,
> >> 
> >> One option is to specify these in the enum:
> >> 
> >> /// The value that can be returned from an IrqHandler or a ThreadedIrqHandler.
> >> pub enum IrqReturn {
> >>    /// The interrupt was not from this device or was not handled.
> >>    None = bindings::irqreturn_IRQ_NONE,
> >> 
> >>    /// The interrupt was handled by this device.
> >>    Handled = bindings::irqreturn_IRQ_HANDLED,
> >> }
> >
> > This requires explicitly setting #[repr(u32)], which is something that was
> > reverted at an earlier iteration of the series on Benno’s request.
> 
> Yeah I requested this, because it increases the size of the enum to 4
> bytes and I think we should try to make rust enums as small as possible.
> 
> @Alice what's the benefit of doing it directly in the enum?

Basically all uses of this enum are going to look like this:

	fn inner() -> IrqReturn {
	    if !should_handle() {
	        return IrqReturn::None;
	    }
	    // .. handle the irq
	    IrqReturn::Handled
	}
	
	fn outer() -> u32 {
	    match inner() {
	        IrqReturn::None => bindings::irqreturn_IRQ_NONE,
	        IrqReturn::Handled => bindings::irqreturn_IRQ_HANDLED,
	    }
	}

Setting the discriminant to the values ensures that the match in outer()
is a no-op. The enum is never going to be stored long-term anywhere so
its size doesn't matter.

Alice