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

["Danilo Krummrich" <dakr@kernel.org>]

On Sun Jul 13, 2025 at 2:16 PM CEST, Benno Lossin wrote:
> 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!`?

Sure, that's possible too.

>>> 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?

RegistrationInner calls free_irq() in it's drop() method; Devres revokes it on
device unbind.

>>
>>   (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).

If we use container_of!() instead or just pass the address of Self (i.e.
Registration) to request_irq() instead,

	pub fn device(&self) -> &Device

is absolutely possible to add to Devres, of course.

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

`&Device<Normal>` in `&Device<Bound>`, yes. I have such a method locally
already (but haven't sent it yet), because that's going to be a use-case for
other abstractions as well. One specific example is the PWM Chip abstraction
[1].

[1] https://lore.kernel.org/lkml/20250710-rust-next-pwm-working-fan-for-sending-v11-3-93824a16f9ec@samsung.com/