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

[Takashi Iwai]

At Tue, 17 Nov 2009 11:59:36 +0100,
Javier Kohen wrote:
> 
> Clemens,
> 
> On Tue, Nov 17, 2009 at 10:04, Clemens Ladisch <clemens at ladisch.de> wrote:
> >
> > Javier Kohen wrote:
> > > 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).
> >
> > I'd guess that Windows ignores the master volume when it finds
> > channel volume controls, so this function was never actually tested.
> >
> > > However, it seems that either the USB specs are wrong,
> >
> > They are abominations written by committees.
> >
> > > or my USB hub or the card are buggy.
> >
> > Your hub is certainly buggy ...
> >
> > > the PCM2702 and my motherboard don't interact well. After a while the
> > > sound is degraded, first into a hiss, then into pure noise.
> >
> > ... the card too.
> >
> >
> > Your patch looks good; please provide a log message and a Signed-off-by
> > tag.
> 
> Thanks for the prompt review!
> 
> Disable the master volume control in the PCM2702 chipset.
> 
> The datasheet documents two independent channel volume controls, one
> master mute control and one master volume control. All controls are
> fully functional except for the master volume control, which returns
> USB stalls on all GET requests.
> 
> Signed-off-by: Javier Kohen <jkohen at users.sourceforge.net>
> ---
> diff --git a/sound/usb/usbmixer.c b/sound/usb/usbmixer.c
> index 9efcfd0..406315e 100644
> --- a/sound/usb/usbmixer.c
> +++ b/sound/usb/usbmixer.c
> @@ -1071,6 +1071,16 @@ static int parse_audio_feature_unit(struct
> mixer_build *state, int unitid, unsig
>  	channels = (ftr[0] - 7) / csize - 1;
> 
>  	master_bits = snd_usb_combine_bytes(ftr + 6, csize);
> +	/* master configuration quirks */
> +	switch (state->chip->usb_id) {
> +	case USB_ID(0x08bb, 0x2702):
> +		snd_printk(KERN_INFO
> +			   "master volume quirk for PCM2702 chip\n");

Better to put a prefix like "usbmixer: master volume ..." since
snd_printk() can be turned to the normal printk() depending on the
kernel config.


thanks,

Takashi