Re: [PATCH 1/2] rust: usb: add basic USB abstractions

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

On Tue Sep 23, 2025 at 4:49 PM CEST, Greg Kroah-Hartman wrote:
> On Tue, Sep 23, 2025 at 04:42:11PM +0200, Danilo Krummrich wrote:
>> On 9/23/25 4:37 PM, Greg Kroah-Hartman wrote:
>> > Yes, you are right, it can be gotten that way.  But I can't wait to see
>> > how you wrap that C macro in rust :)
>> 
>> We can either create a Rust helper function for it, or just re-implement it; in
>> the end it boils down to just a container_of() on the parent device.
>
> Yes, and it preserves the "const" of the pointer going into the function
> call, can we do that in rust as well?

Yes, the Rust container_of!() macro should preserve that.

But despite that, I think it doesn't matter too much in this specific case.

Abstractions of C structures are usually contained within the kernel's Opaque<T>
type, which allows for interior mutability.

Actual mutability is controlled by the corresponding abstraction around the
Opaque<T>.

For instance, a struct device representation looks like this:

	 struct Device<Ctx: DeviceContext = Normal>(Opaque<bindings::device>, PhantomData<Ctx>);

In this case, we never give out a mutable reference to a Device, but rather
control mutability internally with the help of the device context.

For instance, if we have a &Device<Core>, we're guranteed that we're called from
a context where the device_lock() is guaranteed to be held, so we can allow for
some interior mutability.