Re: [PATCH] rust: miscdevice: add registration data to MiscDevice

[Alice Ryhl]

On Fri, Aug 07, 2026 at 04:48:53PM +0100, Gary Guo wrote:
> On Fri Aug 7, 2026 at 4:44 PM BST, Alice Ryhl wrote:
> > On Fri, Aug 7, 2026 at 4:43 PM Gary Guo <gary@xxxxxxxxxxx> wrote:
> >>
> >> On Fri Aug 7, 2026 at 3:22 PM BST, Alice Ryhl wrote:
> >> >  impl<T: MiscDevice> MiscDeviceRegistration<T> {
> >> >      /// Register a misc device.
> >> > -    pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
> >> > +    pub fn register(
> >> > +        opts: MiscDeviceOptions,
> >> > +        data: impl PinInit<T::RegistrationData, Error>,
> >>
> >> We can just skip the `RegistrationData` and use `T`?
> >
> > `T` is the type of the data stored per-fd. We do not separate types
> > for those two concepts.
> 
> This is getting backwards though, and won't work when we need to introduce
> lifetimes.
> 
> Per-fd data needs to be able to reference whatever lifetime is available on
> per-reg data, so it should be assoc type of the per-reg data type. The way
> around you current have causes `T::RegistrationData` to be able to reference
> lifetimes on `T` which is not okay and would only work with `'static` data.

Unfortunately the fds can outlive the registration, so we can't have
such lifetimes to begin with.

I know that miscdevice isn't currently built to work with the new device
model. We should fix that, but it is out of scope for this patch. Until
then, I think the current solution in this patch is sufficient.

Alice