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

[Colin Braun <colinbrauncl@gmail.com>]

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>.