[alsa-devel] usb-audio: Correct way to do a mixer quirk?

[Mark Hills]

On Mon, 20 Feb 2012, Clemens Ladisch wrote:

> Mark Hills wrote:
> > The Electrix Ebox-44 is a new USB interface, but the MIXER_UNIT descriptor
> > is broken and refers to a non-existing unit_id.
> >
> > I worked around it with the patch below to remove the additional entry,
> > and the device seems to work as expected.
> >
> > But inserting code here is probably not desirable -- I imagine I need to
> > turn this into a quirk of some sort. Am I correct?
> >
> > And what is the best way to do this?
> >
> > * It doesn't appear I can use quirks-table.h to adjust the content
> >   of a mixer unit (the bNrInPins attribute)
> >
> > * It seems that to use mixer_quirks.c would require a function to
> >   create the whole mixer
> >
> > It's possible I'm incorrect on the above, though. Can anyone advise on how
> > to make this fix in the correct way?
> 
> I'd suggest to change the code to not abort if a unit doesn't exist.

After changing this code I find myself up against other bad things about 
the desciptor.

The input mixer is not visible. It has a FEATURE_UNIT (10) but referred to 
by a broken MIXER_UNIT (11). There is Maya44 code which stops it being 
found (see below). Removing it and the control is visible. But that does 
not excuse the bad MIXER_UNIT.

Even with the hack, alsamixer presents the 2 volumes as (left, right) 
pairs, when they are actually 2 volumes controlling input pair 1, and 
input pair 2. It would be sensible to present this as 2x mono controls.

I mapped out the description coming from the device. I'm new here, but it 
seems odd that 4[FU] comes under 7[FU], and that outputs seem to be 
inputs. Is this description just badly broken, and is this a problem?

  7[FU] --> 6[MU] --> 5[IT] ("USB streaming")
                  --> 4[FU] --> 3[IT] ("Headphones")
                  --> 2[?]

  11[MU] --> 10[FU] --> 9[IT] ("Line connector")

  8[OT] ("Speaker")

  12[OT] ("USB Streaming")

Where am I best to go next?

eg. should I be trying to write code in mixer_quirks.c to build the 
desired mixer from scratch. And is this even possible to turn the 
left/right controls into mono?

Or is there some better way?

Thanks

-- 
Mark


diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index c374c72..42df7b5 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -1377,11 +1379,6 @@ static int parse_audio_mixer_unit(struct mixer_build *state, int unitid, void *r
                snd_printk(KERN_ERR "invalid MIXER UNIT descriptor %d\n", unitid);
                return -EINVAL;
        }
-       /* no bmControls field (e.g. Maya44) -> ignore */
-       if (desc->bLength <= 10 + input_pins) {
-               snd_printdd(KERN_INFO "MU %d has no bmControls field\n", unitid);
-               return 0;
-       }
 
        num_ins = 0;
        ich = 0;