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Date: Mon, 21 Jul 2025 15:41:34 +0200
Message-Id: <DBHRYM3JB4NK.1QLZ4RA7NVI25@kernel.org>
Subject: Re: [PATCH 2/3] device: rust: expand documentation for Device
Cc: "Alice Ryhl" <aliceryhl@google.com>, <rafael@kernel.org>,
 <ojeda@kernel.org>, <alex.gaynor@gmail.com>, <boqun.feng@gmail.com>,
 <gary@garyguo.net>, <bjorn3_gh@protonmail.com>, <lossin@kernel.org>,
 <a.hindborg@kernel.org>, <tmgross@umich.edu>,
 <rust-for-linux@vger.kernel.org>, <linux-kernel@vger.kernel.org>
To: "Greg KH" <gregkh@linuxfoundation.org>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20250717224806.54763-1-dakr@kernel.org>
 <20250717224806.54763-3-dakr@kernel.org> <aH4juIVmj8euE1CA@google.com>
 <2025072125-twine-curling-db0b@gregkh> <aH4telYyyexiMbjx@google.com>
 <DBHQ2ZDUHLSW.LHWUTFQO1E60@kernel.org>
 <2025072110-buddhism-accompany-0682@gregkh>
In-Reply-To: <2025072110-buddhism-accompany-0682@gregkh>

On Mon Jul 21, 2025 at 3:17 PM CEST, Greg KH wrote:
> On Mon, Jul 21, 2025 at 02:13:14PM +0200, Danilo Krummrich wrote:
>> On Mon Jul 21, 2025 at 2:07 PM CEST, Alice Ryhl wrote:
>> > The connection is that to request an irq you must have a &Device<Bound=
>,
>> > so if you can only obtain a &Device<Bound> to a bus device, then that
>> > means that you can never request an irq for a class device.
>>=20
>> As mentioned in my other reply, a class device never owns resources of a=
 "real"
>> device such as an IRQ.
>>=20
>> A USB device, which represents a real device on a bus, is a bus device, =
in your
>> example the class device is the input device.
>>
>
> Just to confuse things a bit more (sorry), there are also "USB class
> devices" that represent USB devices that use the "generic" USB interface
> to userspace api.  You can find these by searching for
> usb_register_dev().

In this case the struct usb_interface represents a bus device, that can bec=
ome
associated with e.g. a class device of the usbmisc class. :)

So, yeah, there are multiple USB devices, some of them are bus devices, som=
e of
them are class devices.

I'm not sure about the exact topology, but AFAIK USB has different bus devi=
ce
types representing the hierarchy, i.e. a device structure for the entire "r=
eal"
USB device and another one for its interfaces.

> Note, this is different from the drivers/usb/class/ drivers, which
> represent various "USB class protocol" that the USB.org group defines,
> and those talk to userspace through the various common class apis
> depending on the specific device type (input, network, etc.) and are USB
> bus drivers.
>
> Naming is hard, wait until you learn about "usb gadget" a word we had to
> invent to describe the thing :)
>
> thanks,
>
> greg k-h