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

Benno Lossin
2025-07-13T12:16:48Z

On Sun Jul 13, 2025 at 1:57 PM CEST, Danilo Krummrich wrote:
> On Sun Jul 13, 2025 at 1:19 PM CEST, Benno Lossin wrote:
>> 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@kernel.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`?
>
> #[pin_data]
> pub struct Registration<T: Handler + 'static> {
> #[pin]
> inner: Devres<RegistrationInner>,
>
> #[pin]
> handler: T,
>
> /// Pinned because we need address stability so that we can pass a pointer
> /// to the callback.
> #[pin]
> _pin: PhantomPinned,
> }
>
> Currently we pass the address of handler to request_irq(), so this doesn't help,
> hence my proposal to replace the above T with Wrapper (actually Wrapper<T>).

You can just use `container_of!`?

>> Also `Devres` only stores `Device<Normal>`, not `Device<Bound>`...
>
> The Devres instance itself may out-live device unbind, but it ensures that the
> encapsulated data does not, hence it holds a reference count, i.e. ARef<Device>.
>
> Device<Bound> or ARef<Device<Bound>> *never* exists, only &'a Device<Bound>
> within a corresponding scope for which we can guarantee the device is bound.
>
> In the proposed wrapper we can store a NonNull<Device<Bound>> though, because we
> can safely give out a &Device<Bound> in the IRQ's handle() callback. This is
> because:
>
> (1) RegistrationInner is guarded by Devres and guarantees that free_irq() is
> completed *before* the device is unbound.

How does it ensure that?

>
> (2) It is guaranteed that the device pointer is valid because (1) guarantees
> it's even bound and because Devres<RegistrationInner> itself has a
> reference count.

Yeah but I would find it much more natural (and also useful in other
circumstances) if `Devres<T>` would give you access to `Device` (at
least the `Normal` type state).

Depending on how (1) is ensured, we might just need an unsafe function
that turns `Device<Normal>` into `Device<Bound>`.

---
Cheers,
Benno