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

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Message-Id: <DB6QX7AK77DQ.1OOCW0I0VLRXA@kernel.org>
Date: Tue, 08 Jul 2025 16:33:23 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
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 Tue Jul 8, 2025 at 1:49 PM CEST, Alice Ryhl wrote:
> 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.

Hmm in this particular case, I think the optimizer will be able to
remove the additional branch too. But I haven't checked.

I usually give the advice to not explicitly set the discriminants and
let the compiler do it. I don't have a strong opinion on this, but we
should figure out which one is better in which cases.

---
Cheers,
Benno