Re: [RFC PATCH 2/4] rust: usb: add usb host interface and endpoint abstractions

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

On Mon Jul 13, 2026 at 10:03 PM CEST, Colin Braun wrote:
> On Mon, Jul 13, 2026 at 03:22:33PM +0200, Danilo Krummrich wrote:
>> (Cc: Oliver)
>> 
>> On Sun Jul 12, 2026 at 11:07 PM CEST, Colin Braun wrote:
>> > @@ -382,8 +556,8 @@ fn as_ref(&self) -> &device::Device<Ctx> {
>> >      }
>> >  }
>> >  
>> > -impl<Ctx: device::DeviceContext> AsRef<Device> for Interface<Ctx> {
>> > -    fn as_ref(&self) -> &Device {
>> > +impl<Ctx: device::DeviceContext> AsRef<Device<Ctx>> for Interface<Ctx> {
>> > +    fn as_ref(&self) -> &Device<Ctx> {
>> >          // SAFETY: `self.as_raw()` is valid by the type invariants.
>> >          let usb_dev = unsafe { bindings::interface_to_usbdev(self.as_raw()) };
>> 
>> Please see commit f12140f21acb ("rust: usb: don't retain device context for the
>> interface parent").
>> 
>> We can't derive the device context of a USB device from a USB interface. Please
>> also see the device context documentation in [1].
>> 
>> USB device drivers are separate from USB interface drivers, we can't assume that
>> a USB device is bound to a USB device driver just because a USB interface (of
>> that same device) is bound to an USB interface driver.
>> 
>> The same is true from the Core context, which means the device is in a bus
>> device callback, where the device lock is held.
>> 
>> This is also the reason why I keep proposing to only expose simple forwarding
>> helpers on usb::Interface to implement URBs (see also [2] and [3]).
>> 
>> An URB requires either usb::Interface<Bound> or, for a USB device driver,
>> usb::Device<Bound>. But since we can't derive usb::Device<Bound> from
>> usb::Interface<Bound> a simple forwarding helper does the trick.
>
> That makes sense, thank you for pointing this out. I should have taken a
> look at the git log for that line to try to understand its background.
>
> I'll remove the usb::Device<device::Bound>::set_interface() and
> usb::Device<device::Bound>::control_msg() implementations in my next
> revision (since they will no longer be used) and just implement them on
> usb::Interface<device::Bound>.

I'd keep them unsafely on usb::Device and then safely expose forwarding via
usb::Device<Bound> and usb::Interface<Bound> once required.