Re: [PATCH] ALSA: usb-audio: Set the value of potential sticky mixers to maximum

[Takashi Iwai <tiwai@suse.de>]

On Sat, 30 May 2026 21:52:49 +0200,
Rong Zhang wrote:
> 
> It makes no sense to restore the saved value for a sticky mixer, since
> setting any value is a no-op.
> 
> However, in some rare cases, SET_CUR is effective despite GET_CUR always
> returns a constant value. These mixers are not sticky, but there's no
> way to distinguish them. Without any additional information, the best
> thing we can do is to set the mixer value to the maximum before bailing
> out, so that a soft mixer can still reach the maximum hardware volume if
> the mixer turns out to be non-sticky. Meanwhile, all channels must be
> synchronized to prevent imbalance volume.
> 
> Fixes: 86aa1ea1f15c ("ALSA: usb-audio: Do not expose sticky mixers")
> Signed-off-by: Rong Zhang <i@rong.moe>
> ---
>  sound/usb/mixer.c | 33 +++++++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
> index 5fba456eb4a9..fb37bb8ad9a9 100644
> --- a/sound/usb/mixer.c
> +++ b/sound/usb/mixer.c
> @@ -1371,10 +1371,8 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
>  				goto no_checks;
>  
>  			ret = check_sticky_volume_control(cval, minchn, saved);
> -			if (ret < 0) {
> -				snd_usb_set_cur_mix_value(cval, minchn, 0, saved);
> -				return ret;
> -			}
> +			if (ret < 0)
> +				goto sticky;
>  
>  			if (cval->min + cval->res < cval->max)
>  				check_volume_control_res(cval, minchn, saved);
> @@ -1431,6 +1429,33 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
>  	}
>  
>  	return 0;
> +
> +sticky:
> +	/*
> +	 * It makes no sense to restore the saved value for a sticky mixer,
> +	 * since setting any value is a no-op.
> +	 *
> +	 * However, in some rare cases, SET_CUR is effective despite GET_CUR
> +	 * always returns a constant value. These mixers are not sticky, but
> +	 * there's no way to distinguish them. Without any additional
> +	 * information, the best thing we can do is to set the mixer value to
> +	 * the maximum before bailing out, so that a soft mixer can still reach
> +	 * the maximum hardware volume if the mixer turns out to be non-sticky.
> +	 * Meanwhile, all channels must be synchronized to prevent imbalance
> +	 * volume.
> +	 */
> +	if (!cval->cmask) {
> +		snd_usb_set_cur_mix_value(cval, 0, 0, cval->max);
> +	} else {
> +		for (i = 0; i < MAX_CHANNELS; i++) {
> +			idx = 0;
> +			if (cval->cmask & BIT(i)) {
> +				snd_usb_set_cur_mix_value(cval, i + 1, idx, cval->max);
> +				idx++;
> +			}
> +		}
> +	}
> +	return ret;

This change is supposed to be applied only to 7.1, and 7.2 should take
another your series ("ALSA: usb-audio: MIXER_GET_CUR_BROKEN related
changes for 7.2") instead, right?  If so, at merging the latter, I'll
revert this one as a preliminary.


thanks,

Takashi