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

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Jun 23, 2025 at 10:31:16AM -0700, Boqun Feng wrote:
> On Mon, Jun 23, 2025 at 05:26:14PM +0200, Benno Lossin wrote:
> > 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@kernel.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(),
> 
> this is "this" instead of "slot", right?
> 
> >                 )
> >             })?;
> >             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 :)
> > 
> 
> Hmm.. but if request_irq() fails, aren't we going to call `drop` on
> `inner`, which drops the `Devres` which will eventually call
> `RegistrationInner::drop()`? And that's a `free_irq()` without
> `request_irq()` succeeded.
> 

This may however work ;-) Because at `request_irq()` time, all it needs
is ready, and if it fails, `RegistrationInner` won't construct.

    try_pin_init!(&this in Self {
        handler,
        inner: Devres::new(
            dev,
            RegistrationInner {
                // Needs to use `handler` address as cookie, same for
                // request_irq().
                cookie: &raw (*(this.as_ptr().cast()).handler),
                irq: {
                     to_result(unsafe { bindings::request_irq(...) })?;
					 irq
				}
             },
             GFP_KERNEL,
        )?,
        _pin: PhantomPinned
    })

Regards,
Boqun

> Regards,
> Boqun
> 
> > ---
> > Cheers,
> > Benno