[alsa-devel] Quirk to disable master volume control in PCM2702

Javier Kohen jkohen at users.sourceforge.net
Tue Nov 17 00:17:10 CET 2009

[Alan Stern on BCC]

Hi ALSA developers,

I've written the attached patch to disable the master volume control
in this chipset. The datasheet [1] documents two independent channel
volume controls, one master mute control and one master volume
control. However, at least on my device (a Fubar USB DAC), the master
volume control always returns USB stalls when queried for any of its
parameters (min/max/res/cur). All other controls work as expected,
both at the protocol and at the application levels.

Read on for the gory details. Otherwise please take the patch at face
value. I'm ready to make changes if necessary to make it more fitting
to the rest of the code. It would be very appreciated if this can go
into 2.6.32, or in a stable patch shortly after.

Normally I wouldn't give this problem a second thought, since in the
worst case a non-functional control appears in the mixer. Only that
Alan Stern submitted a patch to the USB subsystem during the
development of kernel 2.6.31 that enables the sending of the
Clear-TT-Buffer request to USB hubs on stalls from devices. For some
reason upon receiving the Clear-TT-Buffer the audio goes mute, even
though the kernel keeps sending the audio stream to the device. There
is an ongoing thread on the linux-usb ML where Alan argues that
sending the request is the correct behavior and it shouldn't have the
effect I'm seeing. Since I lack any knowledge of the USB protocol, I
take his word. However, it seems that either the USB specs are wrong,
or my USB hub or the card are buggy.

In the meantime I'm trying to get another hub to compare.
Unfortunately I have to rely on the hub, because the PCM2702 and my
motherboard don't interact well. After a while the sound is degraded,
first into a hiss, then into pure noise. Other users have complained
on different forums; one of the wrote to alsa-user a month ago. I
haven't been able to make any progress to track this issue with usbmon
and ALSA verbose debug messages, to nobody's surprise, since I'm a
complete layman in that area. Any help with this would be appreciated
as well.

[1] http://focus.ti.com/docs/prod/folders/print/pcm2702.html
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