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Date: Mon, 13 Jul 2026 22:09:21 +0200
Message-Id: <DJXPS0HY54SU.3JEDTA04V8HJ0@kernel.org>
Subject: Re: [RFC PATCH 2/4] rust: usb: add usb host interface and endpoint
 abstractions
Cc: "Miguel Ojeda" <ojeda@kernel.org>, "Greg Kroah-Hartman"
 <gregkh@linuxfoundation.org>, "Boqun Feng" <boqun@kernel.org>, "Gary Guo"
 <gary@garyguo.net>, =?utf-8?q?Bj=C3=B6rn_Roy_Baron?=
 <bjorn3_gh@protonmail.com>, "Benno Lossin" <lossin@kernel.org>, "Andreas
 Hindborg" <a.hindborg@kernel.org>, "Alice Ryhl" <aliceryhl@google.com>,
 "Trevor Gross" <tmgross@umich.edu>, "Daniel Almeida"
 <daniel.almeida@collabora.com>, "Tamir Duberstein" <tamird@kernel.org>,
 "Alexandre Courbot" <acourbot@nvidia.com>, =?utf-8?q?Onur_=C3=96zkan?=
 <work@onurozkan.dev>, "Mauro Carvalho Chehab" <mchehab@kernel.org>, "Alan
 Stern" <stern@rowland.harvard.edu>, "Mathias Nyman"
 <mathias.nyman@intel.com>, <linux-kernel@vger.kernel.org>,
 <rust-for-linux@vger.kernel.org>, <linux-usb@vger.kernel.org>,
 <linux-media@vger.kernel.org>, "Colin Braun" <colin.braun.cl@gmail.com>,
 <oneukum@suse.com>
To: "Colin Braun" <colinbrauncl@gmail.com>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20260712-urb-abstraction-v1-v1-0-9fa011634ead@gmail.com>
 <20260712-urb-abstraction-v1-v1-2-9fa011634ead@gmail.com>
 <DJXH4JLX4CES.11S3IZ1V2FTHF@kernel.org> <alVEUR0JrF1ga3S8@pendragon>
In-Reply-To: <alVEUR0JrF1ga3S8@pendragon>

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)
>>=20
>> On Sun Jul 12, 2026 at 11:07 PM CEST, Colin Braun wrote:
>> > @@ -382,8 +556,8 @@ fn as_ref(&self) -> &device::Device<Ctx> {
>> >      }
>> >  }
>> > =20
>> > -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 =3D unsafe { bindings::interface_to_usbdev(self.a=
s_raw()) };
>>=20
>> Please see commit f12140f21acb ("rust: usb: don't retain device context =
for the
>> interface parent").
>>=20
>> We can't derive the device context of a USB device from a USB interface.=
 Please
>> also see the device context documentation in [1].
>>=20
>> USB device drivers are separate from USB interface drivers, we can't ass=
ume that
>> a USB device is bound to a USB device driver just because a USB interfac=
e (of
>> that same device) is bound to an USB interface driver.
>>=20
>> The same is true from the Core context, which means the device is in a b=
us
>> device callback, where the device lock is held.
>>=20
>> This is also the reason why I keep proposing to only expose simple forwa=
rding
>> helpers on usb::Interface to implement URBs (see also [2] and [3]).
>>=20
>> 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.