ALSA: usb-audio: UAC2: support read-only freq control

Some clocks might be read-only, e.g., external clocks (see also
UAC2 4.7.2.1).

In this case, setting the sample frequency will always fail
(even if the rate is equal to the current clock rate),
therefore do not write, but read the value and compare to the
requested rate.
If the clock is read only, avoid reading it twice.

If it doesn't match, return -ENXIO since the clock is invalid for
this configuration.
Signed-off-by: Eldad Zack <eldad@fogrefinery.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/clock.c

@@ -347,6 +347,8 @@ static int set_sample_rate_v2(struct snd_usb_audio *chip, int iface,
 	__le32 data;
 	int err, cur_rate, prev_rate;
 	int clock;
+	bool writeable;
+	struct uac_clock_source_descriptor *cs_desc;
 
 	clock = snd_usb_clock_find_source(chip, fmt->clock, true);
 	if (clock < 0)
@@ -354,20 +356,33 @@ static int set_sample_rate_v2(struct snd_usb_audio *chip, int iface,
 
 	prev_rate = get_sample_rate_v2(chip, iface, fmt->altsetting, clock);
 
+	cs_desc = snd_usb_find_clock_source(chip->ctrl_intf, clock);
+	writeable = uac2_control_is_writeable(cs_desc->bmControls, UAC2_CS_CONTROL_SAM_FREQ - 1);
+	if (writeable) {
 		data = cpu_to_le32(rate);
-	if ((err = snd_usb_ctl_msg(dev, usb_sndctrlpipe(dev, 0), UAC2_CS_CUR,
+		err = snd_usb_ctl_msg(dev, usb_sndctrlpipe(dev, 0), UAC2_CS_CUR,
 				      USB_TYPE_CLASS | USB_RECIP_INTERFACE | USB_DIR_OUT,
 				      UAC2_CS_CONTROL_SAM_FREQ << 8,
 				      snd_usb_ctrl_intf(chip) | (clock << 8),
-				   &data, sizeof(data))) < 0) {
+				      &data, sizeof(data));
+		if (err < 0) {
 			snd_printk(KERN_ERR "%d:%d:%d: cannot set freq %d (v2): err %d\n",
 				   dev->devnum, iface, fmt->altsetting, rate, err);
 			return err;
 		}
 
 		cur_rate = get_sample_rate_v2(chip, iface, fmt->altsetting, clock);
+	} else {
+		cur_rate = prev_rate;
+	}
 
 	if (cur_rate != rate) {
+		if (!writeable) {
+			snd_printk(KERN_WARNING
+				   "%d:%d:%d: freq mismatch (RO clock): req %d, clock runs @%d\n",
+				   dev->devnum, iface, fmt->altsetting, rate, cur_rate);
+			return -ENXIO;
+		}
 		snd_printd(KERN_WARNING
 			   "current rate %d is different from the runtime rate %d\n",
 			   cur_rate, rate);