Re: Using a composite device with kernel drivers and libusb at the same time

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Mar 27, 2024 at 12:33:51PM +0100, Martijn Braam wrote:
> I hope this is an acceptable place to ask this question. I've spend a bit of
> time reverse-engineering the USB protocol for a subset of Blackmagic Design
> video mixers and I have made a userspace implementation to make these
> devices usable in Linux with libusb. This is working great but the roadblock
> I have that I can't find a reasonable solution for is that these devices
> also expose an UVC webcam.
> 
> While my control software is active it's not possible to use the UVC webcam
> functionality of the device because with libusb I have to detach the kernel
> from the interfaces to make my userspace access work. As far as I can find
> documented online there's no way to have half a composite device handled by
> kernel drivers and half with userspace drivers in Linux. It seems to me the
> only solution is to make a kernel driver to bind to the control interface
> and have that kernel driver pass-through to userspace somehow with a custom
> protocol? If that is the case would such a driver even be supported in
> Linux?

Why not just make a kernel driver for the interfaces that are not uvc?
We would accept that given that you can not control a device from usbfs
and a driver at the same time as you have pointed out.

thanks,

greg k-h