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

[Takashi Iwai]

On Wed, 21 Aug 2019 12:02:25 +0200,
Srinivas Kandagatla wrote:
> 
> From: Deepa Madiregama <dmadireg at codeaurora.org>
> 
> - 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,

Takashi


> Signed-off-by: Deepa Madiregama <dmadireg at codeaurora.org>
> Signed-off-by: Banajit Goswami <bgoswami at codeaurora.org>
> Signed-off-by: Meng Wang <mwang at codeaurora.org>
> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla at linaro.org>
> ---
>  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
>