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

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

On Mon Jul 14, 2025 at 9:57 AM CEST, Dirk Behme wrote:
> On 13/07/2025 16:19, Danilo Krummrich wrote:
>> On Sun Jul 13, 2025 at 4:09 PM CEST, Daniel Almeida wrote:
>>> On a second look, I wonder how useful this will be.
>>>
>>>  fn handle(&self, dev: &Device<Bound>) -> IrqReturn
>>>
>>> Sorry for borrowing this terminology, but here we offer Device<Bound>, while I
>>> suspect that most drivers will be looking for the most derived Device type
>>> instead. So for drm drivers this will be drm::Device, for example, not the base
>>> dev::Device type. I assume that this pattern will hold for other subsystems as
>>> well.
>>>
>>> Which brings me to my second point: drivers can store an ARef<drm::Device> on
>>> the handler itself, and I assume that the same will be possible in other
>>> subsystems.
>> 
>> Well, the whole point is that you can use a &Device<Bound> to directly access
>> device resources without any overhead, i.e.
>> 
>> 	fn handle(&self, dev: &Device<Bound>) -> IrqReturn {
>> 	   let io = self.iomem.access(dev);
>> 
>> 	   io.write32(...);
>
> As this is exactly the example I was discussing privately with Daniel
> (many thanks!), independent on the device discussion here, just for my
> understanding:
>
> Is it ok to do a 'self.iomem.access(dev)' at each interrupt?

Absolutely, Devres::access() is a very cheap accessor, see also [1]. Compiled
down, the only thing that Revocable::access() does is deriving a pointer from
another pointer by adding an offset.

That's exactly why we want the &Device<Bound> cookie, to avoid more expensive
operations.

[1] https://rust.docs.kernel.org/src/kernel/revocable.rs.html#151

> Wouldn't it
> be cheaper/faster to pass 'io' instead of 'iomem' to the interrupt handler?

Well, consider the types of the example:

	iomem: Devres<IoMem<SIZE>>
	io: &IoMem<Size>

You can't store a reference with a non-static lifetime in something with an
open-ended lifetime, such as the Handler object.

How would you ensure that the reference is still valid? The Devres<IoMem<SIZE>>
object might have been dropped already, either by the user or by Devres revoking
the inner object due to device unbind.