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

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Message-Id: <DBAVXQTMR38Z.2782EGR84L7OP@kernel.org>
Date: Sun, 13 Jul 2025 13:19:51 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Danilo Krummrich" <dakr@...nel.org>, "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>, "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>, "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 Sun Jul 13, 2025 at 12:24 PM CEST, Danilo Krummrich wrote:
> On Sun Jul 13, 2025 at 1:32 AM CEST, Daniel Almeida wrote:
>>
>>
>>> On 12 Jul 2025, at 18:24, Danilo Krummrich <dakr@...nel.org> wrote:
>>> 
>>> On Thu Jul 3, 2025 at 9:30 PM CEST, Daniel Almeida wrote:
>>>> +/// Callbacks for an IRQ handler.
>>>> +pub trait Handler: Sync {
>>>> +    /// The hard IRQ handler.
>>>> +    ///
>>>> +    /// This is executed in interrupt context, hence all corresponding
>>>> +    /// limitations do apply.
>>>> +    ///
>>>> +    /// All work that does not necessarily need to be executed from
>>>> +    /// interrupt context, should be deferred to a threaded handler.
>>>> +    /// See also [`ThreadedRegistration`].
>>>> +    fn handle(&self) -> IrqReturn;
>>>> +}
>>> 
>>> One thing I forgot, the IRQ handlers should have a &Device<Bound> argument,
>>> i.e.:
>>> 
>>> fn handle(&self, dev: &Device<Bound>) -> IrqReturn
>>> 
>>> IRQ registrations naturally give us this guarantee, so we should take advantage
>>> of that.
>>> 
>>> - Danilo
>>
>> Hi Danilo,
>>
>> I do not immediately see a way to get a Device<Bound> from here:
>>
>> unsafe extern "C" fn handle_irq_callback<T: Handler>(_irq: i32, ptr: *mut c_void) -> c_uint {
>>
>> Refall that we've established `ptr` to be the address of the handler. This
>> came after some back and forth and after the extensive discussion that Benno
>> and Boqun had w.r.t to pinning in request_irq().
>
> You can just wrap the Handler in a new type and store the pointer there:
>
> 	#[pin_data]
> 	struct Wrapper {
> 	   #[pin]
> 	   handler: T,
> 	   dev: NonNull<Device<Bound>>,
> 	}
>
> And then pass a pointer to the Wrapper field to request_irq();
> handle_irq_callback() can construct a &T and a &Device<Bound> from this.
>
> Note that storing a device pointer, without its own reference count, is
> perfectly fine, since inner (Devres<RegistrationInner>) already holds a
> reference to the device and guarantees the bound scope for the handler
> callbacks.

Can't we just add an accessor function to `Devres`?

Also `Devres` only stores `Device<Normal>`, not `Device<Bound>`...

---
Cheers,
Benno

> It makes sense to document this as an invariant of Wrapper (or whatever we end
> up calling it).