Re: [PATCH] ALSA: usb-audio: Fix the mixer control range limiting issue

[Srinivas Kandagatla]

On 21/08/2019 18:05, Takashi Iwai wrote:
> On Wed, 21 Aug 2019 12:02:25 +0200,
> Srinivas Kandagatla wrote:
> > From: Deepa Madiregama <dmadireg@xxxxxxxxxxxxxx>
> >
> > - mixer_ctl_set() function is limiting the volume level
> >    to particular range. This results in incorrect initial
> >    volume setting for that device.
> > - In USB mixer while calculating the dBmin/dBmax values
> >    resolution factor is hardcoded to 256 which results in
> >    populating the wrong values for dBmin/dBmax.
> > - Fix is to use appropriate resolution factor while
> >    calculating the dBmin/dBmax values.
>
> This change doesn't sound right.  Basically the values returned from
> USB-audio FEATURE UNIT or MIXER UNIT are always in 1/256 dB unit, per
> definition.  And we pass dB min/max to user-space as TLV_DB_MINMAX(),
> i.e. TLV points just both min and max, no matter what scale is.  I
> believe that the current code is correct in this regard.
>
> So, it's either a firmware bug that gives the wrong values back, or
> the case we still don't cover, e.g. multiple RANGE values for
> UAC2/UAC3.
>
> Please check what exactly doesn't work as expected.  Which value is
> returned from the USB-audio device and what is wrongly interpreted.

Thanks for the detailed review.

I see https://www.usb.org/sites/default/files/audio10.pdf clearly 
specifies Mixer unit is always 1/256dB units.

This patch has been carried in Qcom downstream kernels for past many 
years to fix very low Initial volume setting on some USB headsets like 
Logitech S-150 USB Speakers and Nokia HS-82 USB headset.

Its highly likely that its a firmware bug!, but I will try to get it 
retested on latest kernel and collect some logs!

Thanks,
srini


> thanks,
>
> Takashi
>
>
> > Signed-off-by: Deepa Madiregama <dmadireg@xxxxxxxxxxxxxx>
> > Signed-off-by: Banajit Goswami <bgoswami@xxxxxxxxxxxxxx>
> > Signed-off-by: Meng Wang <mwang@xxxxxxxxxxxxxx>
> > Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@xxxxxxxxxx>
> > ---
> >   sound/usb/mixer.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
> > index 5070a6a76ab3..a67448327d07 100644
> > --- a/sound/usb/mixer.c
> > +++ b/sound/usb/mixer.c
> > @@ -1248,8 +1248,10 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
> >   	/* USB descriptions contain the dB scale in 1/256 dB unit
> >   	 * while ALSA TLV contains in 1/100 dB unit
> >   	 */
> > -	cval->dBmin = (convert_signed_value(cval, cval->min) * 100) / 256;
> > -	cval->dBmax = (convert_signed_value(cval, cval->max) * 100) / 256;
> > +	cval->dBmin =
> > +		(convert_signed_value(cval, cval->min) * 100) / (cval->res);
> > +	cval->dBmax =
> > +		(convert_signed_value(cval, cval->max) * 100) / (cval->res);
> >   	if (cval->dBmin > cval->dBmax) {
> >   		/* something is wrong; assume it's either from/to 0dB */
> >   		if (cval->dBmin < 0)
> > --
> > 2.21.0