[alsa-devel] Resend: Incorrect sbits for snd-usb-audio

[Takashi Iwai]

At Fri, 6 Dec 2013 14:57:46 -0600,
Sander Jansen wrote:
> 
> Hi,
> 
> I had send this out earlier back in September, but never got a response.
> 
> I have a Focusrite Scarlett 2i4 USB Audio Interface. The audio format
> exposed by ALSA for  this device is S32_LE. From the hardware specs and
> lsusb. it supposed to only have a 24 bit resolution (with the LSB bits set
> to 0):
> 
> AudioStreaming Interface Descriptor:
>         bLength                 6
>         bDescriptorType        36
>         bDescriptorSubtype      2 (FORMAT_TYPE)
>         bFormatType             1 (FORMAT_TYPE_I)
>         bSubslotSize            4
>         bBitResolution         24
> 
> When I query the sbits for this device, I expect it to return 24, but
> instead it returns 32. Looking through the usb audio driver code, it
> doesn't look like this information is actually exposed to userspace. The
> 'parse_audio_format_i_type'  in usb/format.c merely returns the supported
> formats, but doesn't include any msbits info. (and it doesn't look like it
> can be easily changed to return the msbits info as well).
> 
> Was this intentionally left out, or is this a missing feature in the driver?

It's just because of laziness because almost no applications check the
msbit but only the format type.  Feel free to send an enhancement
patch.


thanks,

Takashi