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

[Daniel Almeida <daniel.almeida@collabora.com>]

> 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:
>> 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
>>    })
> 
> 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.



— Daniel