ALSA: usb-audio: Add a more accurate volume quirk for AudioQuest DragonFly

commit 42e3121d90f42e57f6dbd6083dff2f57b3ec7daa upstream.

AudioQuest DragonFly DAC reports a volume control range of 0..50
(0x0000..0x0032) which in USB Audio means a range of 0 .. 0.2dB, which
is obviously incorrect and would cause software using the dB information
in e.g. volume sliders to have a massive volume difference in 100..102%
range.

Commit 2d1cb7f658fb ("ALSA: usb-audio: add dB range mapping for some
devices") added a dB range mapping for it with range 0..50 dB.

However, the actual volume mapping seems to be neither linear volume nor
linear dB scale, but instead quite close to the cubic mapping e.g.
alsamixer uses, with a range of approx. -53...0 dB.

Replace the previous quirk with a custom dB mapping based on some basic
output measurements, using a 10-item range TLV (which will still fit in
alsa-lib MAX_TLV_RANGE_SIZE).

Tested on AudioQuest DragonFly HW v1.2. The quirk is only applied if the
range is 0..50, so if this gets fixed/changed in later HW revisions it
will no longer be applied.

v2: incorporated Takashi Iwai's suggestion for the quirk application
method

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2: open-code usb_audio_info()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index 488e531..2dc90a4 100644 (file)
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -1213,6 +1213,8 @@ static void build_feature_ctl(struct mixer_build *state, void *raw_desc,
                break;
        }
 
+       snd_usb_mixer_fu_apply_quirk(state->mixer, cval, unitid, kctl);
+
        range = (cval->max - cval->min) / cval->res;
        /* Are there devices with volume range more than 255? I use a bit more
         * to be sure. 384 is a resolution magic number found on Logitech

diff --cc sound/usb/mixer_maps.c
Simple merge

diff --cc sound/usb/mixer_quirks.c
Simple merge

diff --cc sound/usb/mixer_quirks.h
Simple merge