[alsa-devel] [RFC] ALSA: usb-audio: Add custom mixer status quirks for RME CC devices

[Takashi Iwai]

On Fri, 21 Sep 2018 00:07:19 +0200,
Jussi Laako wrote:
> 
> This patch accessing vendor specific registers of RME Class Compliant
> devices works otherwise fine, but there is still one small problem. It is
> supposed to be a polled from the hardware, since the values indicate
> device status and are subject to change at all times. Behavior is correct
> if I keep running "amixer" all the time to retrieve values - sampling rate
> changes are visible and values get updated. However, when I poll the
> values from an application without reopening the mixer, they are being
> cached somewhere and the changes are not reflected. This problem doesn't
> exist for example with PCIe based RME HDSPe AIO and application gets
> updated values, but in this USB based device case they don't get updated.
> I have flagged all the mixer controls as read-only and volatile, but the
> volatile flag is not honored in this (assuming I've understood it correctly).
> 
> Any guidance on how to fix the caching problem is welcome!

The caching is currently enabled for all elements, but changing it
should be trivial.  The patch below adds is_volatile flag to the
element, and you can set it to true in the quirk somehow for uncached
controls.


thanks,

Takashi

---
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -449,8 +449,10 @@ int snd_usb_get_cur_mix_value(struct usb_mixer_elem_info *cval,
 				      cval->control, channel, err);
 		return err;
 	}
-	cval->cached |= 1 << channel;
-	cval->cache_val[index] = *value;
+	if (!cval->is_volatile) {
+		cval->cached |= 1 << channel;
+		cval->cache_val[index] = *value;
+	}
 	return 0;
 }
 
@@ -540,8 +542,10 @@ int snd_usb_set_cur_mix_value(struct usb_mixer_elem_info *cval, int channel,
 					  value);
 	if (err < 0)
 		return err;
-	cval->cached |= 1 << channel;
-	cval->cache_val[index] = value;
+	if (!cval->is_volatile) {
+		cval->cached |= 1 << channel;
+		cval->cache_val[index] = value;
+	}
 	return 0;
 }
 
diff --git a/sound/usb/mixer.h b/sound/usb/mixer.h
index 3d12af8bf191..cfbc274ce89f 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -70,6 +70,7 @@ struct usb_mixer_elem_info {
 	int val_type;
 	int min, max, res;
 	int dBmin, dBmax;
+	bool is_volatile;
 	int cached;
 	int cache_val[MAX_CHANNELS];
 	u8 initialized;