Re: Two devices, same USB ID: one needs HID, the other doesn't. How to solve this?

On 13.01.2012 12:42, Hans Verkuil wrote:

Hi!

Hi!

Adding Jiri Kosina, the HID maintainer.

I've made a video4linux driver for the USB Keene FM Transmitter. See:

http://www.amazon.co.uk/Keene-Electronics-USB-FM-Transmitter/dp/B003GCHPDY/ref=sr_1_1?ie=UTF8&qid=1326450476&sr=8-1

The driver code is here:

http://git.linuxtv.org/hverkuil/media_tree.git/shortlog/refs/heads/keene

Unfortunately this device has exactly the same USB ID as the Logitech AudioHub
USB speaker (http://www.logitech.com/en-us/439/3503).

The AudioHub has HID support for volume keys, but the FM transmitter needs
a custom V4L2 driver instead.

I've attached the full lsusb -v output of both devices, but this is the diff of
the two:

$ diff keene.txt audiohub.txt -u

[...]

The HID Report descriptors could be interesting as they differ. You can
look at them in:
/sys/kernel/debug/hid/*/rdesc

I guess one option would be to make this a "regular" HID driver like
those in drivers/hid/hid-*.c (and just set the v4l things up if the
descriptor is as expected, otherwise let standard HID-input handle
them), but there is the issue of where to place the driver, then, as it
can't be both in drivers/hid and drivers/media...

Probably the easy way out is to simply add the device into
drivers/hid/hid-core.c:hid_ignore(), by checking e.g.
vendor+product+name, and hope all "B-LINK USB Audio" devices are FM
transmitters (the name suggests that may not necessarily be the case,
though). Report descriptor contents are not available at hid_ignore()
point yet.

In my git tree I worked around it by adding the USB ID to the ignore list
if the Keene driver is enabled, and ensuring that the Keene driver is
disabled by default.

But is there a better method to do this? At least the iProduct strings are
different, is that something that can be tested in hid-core.c?

Regards,

	Hans

-- 
Anssi Hannula