Re: [PATCH 0/3] ALSA: snd-usb: Some small fixes to make Playback Design products work

[Daniel Mack <zonque@gmail.com>]

On 18.03.2013 10:07, Takashi Iwai wrote:
> At Sun, 17 Mar 2013 20:07:23 +0800,
> Daniel Mack wrote:
>>
>> Thanks to Andreas, I got my hands on a Playback Design MPD-3 and hacked
>> up some small hanges necessary to make it fully work with snd-usb.
>>
>> The device reports 0x80000000 in the bmFormats field of two of three of
>> its alternate settings, which wrongly made the driver believe it's a
>> usual PCM endpoint (actually due to a fix-up fallback). However, bit 31
>> of this mask in fact denotes 'raw data'.
>>
>> The effect of this issue is that in addition to the first altsetting with
>> a bit depth of 24, the driver exposed 8-bit and 16-bit native audio
>> formats on the raw data endpoints, which do not work as expected.
>>
>> Also, those devices need a 50ms delay after switching the USB interface.
> 
> Thanks, applied to for-next branch.
> 
> The remaining question is which PCM format is suitable for the raw
> data.  SND_PCM_FORMAT_SPECIAL should be OK, it won't regress at least,
> as no apps support this format.

Yes, I  thought about that for a while as well. In fact, the data format
the device supports on these altsettings is "DSD", but that doesn't have
a defined value in the UAC2 spec (and strictly speaking, DSD is not even
PCM).

So when a device is as unspecific as 'raw data', there's not much we can
do about that except for exposing the same level of uncertainty down to
the apps, right?


Thanks,
Daniel