[alsa-devel] usb-audio: I'd like to change two things in usb audio, but I'm not sure if it's right

[Johan Aires Rastén]

So I'll try again, this time with the proper cc:s and a second question.

I've come across two issues in the usb audio module that are causing
problems with my sound card, but I'm thinking that changing them might
break something for some other card.

In sound/usb/mixer.c function check_input_term(...), for the case
UAC_INPUT_TERMINAL and UAC_VERSION_2:

The recursive call to get clock selectors messes up the naming of my
terminals. At this stage the terminals are already properly named Mic and
Line but the call overwrites them both with the unnamed clock selector
"Unit 22". Are other UAC2 cards different, so that the clock selector
contains the correct terminal name, or is there some other reasoning here
that I'm not seeing?

Could the call be removed, or perhaps only done if no terminal name has
been found yet?

In sound/usb/stream.c function snd_usb_add_audio_stream(...), near the end
of the function:

A newly created pcm is added with "list_add(&as->list, &chip->pcm_list);"
which causes devices to be listed in reverse order, i.e. device 0 will be
at the end of the list, and this leads to that capture streams end up in
the wrong device under certain circumstances.

The problem arises when the code under "look for an empty stream" earlier
in the same function executes. It will attach the capture stream to the
first compatible existing stream, but since they are reversed it will find
the highest numbered device. In my case analog input will be grouped with
digital out (device 1) instead of analog out (device 0).

This is assuming that playback streams are defined before capture streams
but it could happen in the other direction as long as there are several
streams of one type.

I also suspect that this is what causes the M-Audio Audiophile USB to have
swapped inputs, for which there's a fix in USB-Audio.conf (
http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/conf/cards/USB-Audio.conf#l70
). Thanks to debianuser on IRC for helping me find it.

I would like to change this call to use list_add_tail for adding new items
instead, but I'm worried that there are quirks or other fixes that expect
the current behaviour. There are very few other references to pcm_list and
none of them seem to care about ordering.

Sorry for being bad at referencing code, I'm new at this.

//Johan Rastén