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

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

On Tue, Jun 24, 2025 at 07:33:35AM -0700, Boqun Feng wrote:
> On Tue, Jun 24, 2025 at 02:50:23PM +0100, Alice Ryhl wrote:
> > On Tue, Jun 24, 2025 at 1:46 PM Benno Lossin <lossin@kernel.org> wrote:
> > >
> > > On Tue Jun 24, 2025 at 2:31 PM CEST, Daniel Almeida wrote:
> > > > On 23 Jun 2025, at 16:28, Benno Lossin <lossin@kernel.org> wrote:
> > > >> On Mon Jun 23, 2025 at 9:18 PM CEST, Boqun Feng wrote:
> > > >>>    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
> > > >>>    })
> > > >>
> > > >> Well yes and no, with the Devres changes, the `cookie` can just be the
> > > >> address of the `RegistrationInner` & we can do it this way :)
> > > >>
> > > >> ---
> > > >> Cheers,
> > > >> Benno
> > > >
> > > >
> > > > No, we need this to be the address of the the whole thing (i.e.
> > > > Registration<T>), otherwise you can’t access the handler in the irq
> > > > callback.
> 
> You only need the access of `handler` in the irq callback, right? I.e.
> passing the address of `handler` would suffice (of course you need
> to change the irq callback as well).
> 
> > >
> > > Gotcha, so you keep the cookie field, but you should still be able to
> > > use `try_pin_init` & the devres improvements to avoid the use of
> > > `pin_init_from_closure`.
> > 
> > It sounds like this is getting too complicated and that
> > `pin_init_from_closure` is the simpler way to go.
> 
> Even if we use `pin_init_from_closure`, we still need the other
> `try_pin_init` anyway for `Devres::new()` (or alternatively we can
> implement a `RegistrationInner::new()`).
> 
> Below is what would look like with the Devres changes in mind:
> 
> 
>     try_pin_init!(&this in Self {
>         handler,
>         inner: <- Devres::new(
>             dev,
>             try_pin_init!( RegistrationInner {
>                 // Needs to use `handler` address as cookie, same for
>                 // request_irq().
>                 cookie: &raw (*(this.as_ptr().cast()).handler),
> 		// @Benno, would this "this" work here?
>                 irq: {
>                      to_result(unsafe { bindings::request_irq(...) })?;
>                      irq
> 		}
>              }),
>         )?,
>         _pin: PhantomPinned
>     })
> 
> 

Never mind, `dev` is a `Device<Bound>` so it cannot be unbounded during
the call ;-)

Regards,
Boqun

> Besides, working on this made me realize that we have to request_irq()
> before `Devres::new()`, otherwise we may leak the irq resource,
> considering the follow code from the current `pin_init_from_closure`
> approach:
> 
>         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,
>                 })
>             };
> 
> `dev` can be unbound at here, right? If so, the devm callback will
> revoke the `RegistrationInner`, `RegistrationInner::drop()` will then
> call `free_irq()` before `request_irq()`, the best case is that we would
> request_irq() with no one going to free it.
> 
>             // 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(),
>                 )
>             });
>             ...
>         }
> 
> So seems to me the order of initialization has to be:
> 
> 1. Initialize the `handler`.
> 2. `request_irq()`, i.e initialize the `RegistrationInner`.
> 3. `Devres::new()`, i.e initialize the `Devres`.
> 
> Regards,
> Boqun