linux-kernel - Re: [PATCH v4 3/6] rust: irq: add support for non-threaded IRQs and handlers

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Message-Id: <DAU0NHTHTDG4.2HNEABNAI8GHZ@kernel.org>
Date: Mon, 23 Jun 2025 17:26:14 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "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>, "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>, "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 v4 3/6] rust: irq: add support for non-threaded IRQs and
 handlers

On Mon Jun 23, 2025 at 5:10 PM CEST, Alice Ryhl wrote:
> On Mon, Jun 9, 2025 at 12:47 PM Danilo Krummrich <dakr@...nel.org> wrote:
>> On Sun, Jun 08, 2025 at 07:51:08PM -0300, Daniel Almeida wrote:
>> > +        dev: &'a Device<Bound>,
>> > +        irq: u32,
>> > +        flags: Flags,
>> > +        name: &'static CStr,
>> > +        handler: T,
>> > +    ) -> impl PinInit<Self, Error> + 'a {
>> > +        let closure = move |slot: *mut Self| {
>> > +            // SAFETY: The slot passed to pin initializer is valid for writing.
>> > +            unsafe {
>> > +                slot.write(Self {
>> > +                    inner: Devres::new(
>> > +                        dev,
>> > +                        RegistrationInner {
>> > +                            irq,
>> > +                            cookie: slot.cast(),
>> > +                        },
>> > +                        GFP_KERNEL,
>> > +                    )?,
>> > +                    handler,
>> > +                    _pin: PhantomPinned,
>> > +                })
>> > +            };
>> > +
>> > +            // SAFETY:
>> > +            // - The callbacks are valid for use with request_irq.
>> > +            // - If this succeeds, the slot is guaranteed to be valid until the
>> > +            // destructor of Self runs, which will deregister the callbacks
>> > +            // before the memory location becomes invalid.
>> > +            let res = to_result(unsafe {
>> > +                bindings::request_irq(
>> > +                    irq,
>> > +                    Some(handle_irq_callback::<T>),
>> > +                    flags.into_inner() as usize,
>> > +                    name.as_char_ptr(),
>> > +                    slot.cast(),
>> > +                )
>> > +            });
>> > +
>> > +            if res.is_err() {
>> > +                // SAFETY: We are returning an error, so we can destroy the slot.
>> > +                unsafe { core::ptr::drop_in_place(&raw mut (*slot).handler) };
>> > +            }
>> > +
>> > +            res
>> > +        };
>> > +
>> > +        // SAFETY:
>> > +        // - if this returns Ok, then every field of `slot` is fully
>> > +        // initialized.
>> > +        // - if this returns an error, then the slot does not need to remain
>> > +        // valid.
>> > +        unsafe { pin_init_from_closure(closure) }
>>
>> Can't we use try_pin_init!() instead, move request_irq() into the initializer of
>> RegistrationInner and initialize inner last?
>
> We need a pointer to the entire struct when calling
> bindings::request_irq. I'm not sure this allows you to easily get one?
> I don't think using container_of! here is worth it.

There is the `&this in` syntax (`this` is of type `NonNull<Self>`):

    try_pin_init!(&this in Self {
        inner: Devres::new(
            dev,
            RegistrationInner {
                irq,
                cookie: this.as_ptr().cast(),
            },
            GFP_KERNEL,
        )?,
        handler,
        _pin: {
            to_result(unsafe {
                bindings::request_irq(
                    irq,
                    Some(handle_irq_callback::<T>),
                    flags.into_inner() as usize,
                    name.as_char_ptr(),
                    slot.as_ptr().cast(),
                )
            })?;
            PhantomPinned
        },
    })

Last time around, I also asked this question and you replied with that
we need to abort the initializer when `request_irq` returns false and
avoid running `Self::drop` (thus we can't do it using `pin_chain`).

I asked what we could do instead and you mentioned the `_: {}`
initializers and those would indeed solve it, but we can abuse the
`_pin` field for that :)

---
Cheers,
Benno