[PATCH 2/3] ALSA: usb-audio: Add QUIRK_FLAG_MIXER_SKIP_GET_CUR_VOL

[Rong Zhang]

Since commit 86aa1ea1f15c ("ALSA: usb-audio: Do not expose sticky
mixers"), the UAC mixer core utilizes volume SET_CUR and GET_CUR to
identify devices with sticky mixers. Unfortunately, even though most
devices with sticky GET_CUR also have corresponding sticky SET_CUR,
which I actually met more since the commit had been merged, there is
also a rare case that some devices may have volume mixers that responds
to SET_CUR properly but with its GET_CUR stubbed. This cause the sticky
check to consider the mixer to be sticky and unnecessarily disable it.

Add QUIRK_FLAG_MIXER_SKIP_GET_CUR_VOL to prevent sending GET_CUR to
mixers by returning -ENXIO early. The error effectively skips the sticky
check as it's only meaningful when the mixer has some sort of self-
awareness. Similar to QUIRK_FLAG_GET_SAMPLE_RATE, this should also help
if some unmet devices can't tolerate volume GET_CUR in other ways.

Signed-off-by: Rong Zhang <i@xxxxxxxx>
---
 Documentation/sound/alsa-configuration.rst | 4 ++++
 sound/usb/mixer.c                          | 5 +++++
 sound/usb/quirks.c                         | 1 +
 sound/usb/usbaudio.h                       | 6 ++++++
 4 files changed, 16 insertions(+)

diff --git a/Documentation/sound/alsa-configuration.rst b/Documentation/sound/alsa-configuration.rst
index 4b30cd63c5a5..bc3bc65c379a 100644
--- a/Documentation/sound/alsa-configuration.rst
+++ b/Documentation/sound/alsa-configuration.rst
@@ -2389,6 +2389,10 @@ quirk_flags
           from snd_usb_handle_sync_urb. Instead fall through and enqueue a
           packet_info containing only size-0 packets, so the OUT ring keeps
           moving (emits silence). Needed by Behringer Flow 8 (1397:050c).
+        * bit 30: ``mixer_skip_get_cur_vol``
+          Skip reading current volume for mixers, as some devices return
+          constant values or errors but otherwise works fine, i.e., setting
+          volume takes desired effect.
 
 This module supports multiple devices, autoprobe and hotplugging.
 
diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index d61bde654219..3b745aebb181 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -420,6 +420,11 @@ static int get_cur_ctl_value(struct usb_mixer_elem_info *cval,
 static inline int get_cur_mix_raw(struct usb_mixer_elem_info *cval,
 				  int channel, int *value)
 {
+	struct snd_usb_audio *chip = cval->head.mixer->chip;
+
+	if (chip->quirk_flags & QUIRK_FLAG_MIXER_SKIP_GET_CUR_VOL)
+		return -ENXIO;
+
 	return get_ctl_value(cval, UAC_GET_CUR,
 			     (cval->control << 8) | channel,
 			     value);
diff --git a/sound/usb/quirks.c b/sound/usb/quirks.c
index e2c95be38aca..fbd44baace68 100644
--- a/sound/usb/quirks.c
+++ b/sound/usb/quirks.c
@@ -2605,6 +2605,7 @@ static const char *const snd_usb_audio_quirk_flag_names[] = {
 	QUIRK_STRING_ENTRY(MIXER_PLAYBACK_LINEAR_VOL),
 	QUIRK_STRING_ENTRY(MIXER_CAPTURE_LINEAR_VOL),
 	QUIRK_STRING_ENTRY(IFB_SILENCE_ON_EMPTY),
+	QUIRK_STRING_ENTRY(MIXER_SKIP_GET_CUR_VOL),
 	NULL
 };
 
diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index 9afcad8f143a..f4e048788f0b 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -242,6 +242,10 @@ extern bool snd_usb_skip_validation;
  *  from snd_usb_handle_sync_urb. Instead fall through and enqueue a
  *  packet_info containing only size-0 packets, so the OUT ring keeps
  *  moving (emits silence). Needed by Behringer Flow 8 (1397:050c).
+ * QUIRK_FLAG_MIXER_SKIP_GET_CUR_VOL
+ *  Skip reading current volume for mixers, as some devices return constant
+ *  values or errors but otherwise works fine, i.e., setting volume takes
+ *  desired effect.
  */
 
 enum {
@@ -275,6 +279,7 @@ enum {
 	QUIRK_TYPE_MIXER_PLAYBACK_LINEAR_VOL	= 27,
 	QUIRK_TYPE_MIXER_CAPTURE_LINEAR_VOL	= 28,
 	QUIRK_TYPE_IFB_SILENCE_ON_EMPTY		= 29,
+	QUIRK_TYPE_MIXER_SKIP_GET_CUR_VOL	= 30,
 /* Please also edit snd_usb_audio_quirk_flag_names */
 };
 
@@ -310,5 +315,6 @@ enum {
 #define QUIRK_FLAG_MIXER_PLAYBACK_LINEAR_VOL	QUIRK_FLAG(MIXER_PLAYBACK_LINEAR_VOL)
 #define QUIRK_FLAG_MIXER_CAPTURE_LINEAR_VOL	QUIRK_FLAG(MIXER_CAPTURE_LINEAR_VOL)
 #define QUIRK_FLAG_IFB_SILENCE_ON_EMPTY		QUIRK_FLAG(IFB_SILENCE_ON_EMPTY)
+#define QUIRK_FLAG_MIXER_SKIP_GET_CUR_VOL	QUIRK_FLAG(MIXER_SKIP_GET_CUR_VOL)
 
 #endif /* __USBAUDIO_H */

-- 
2.53.0