[PATCH 10/10] ALSA: usb-audio: UAC2: support read-only freq control

[Eldad Zack <eldad@fogrefinery.com>]

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 it doesn't match, return -ENXIO since the clock is invalid for
this configuration.

Signed-off-by: Eldad Zack <eldad@fogrefinery.com>
---
 sound/usb/clock.c | 39 +++++++++++++++++++++++++++------------
 1 file changed, 27 insertions(+), 12 deletions(-)

diff --git a/sound/usb/clock.c b/sound/usb/clock.c
index 1c0ec82..6ad9951 100644
--- a/sound/usb/clock.c
+++ b/sound/usb/clock.c
@@ -325,35 +325,50 @@ static int set_sample_rate_v2(struct snd_usb_audio *chip, int iface,
 	u32 data;
 	int err, crate;
 	int clock;
+	bool writeable;
+	struct uac_clock_source_descriptor *cs_desc;
 
 	clock = snd_usb_clock_find_source(chip, fmt->clock, true);
 	if (clock < 0)
 		return clock;
 
-	data = cpu_to_le32(rate);
-	if ((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) {
-		snd_printk(KERN_ERR "%d:%d:%d: cannot set freq %d (v2): err %d\n",
-			   dev->devnum, iface, fmt->altsetting, rate, err);
-		return err;
+	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);
+		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));
+		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;
+		}
 	}
 
-	if ((err = snd_usb_ctl_msg(dev, usb_rcvctrlpipe(dev, 0), UAC2_CS_CUR,
+	err = snd_usb_ctl_msg(dev, usb_rcvctrlpipe(dev, 0), UAC2_CS_CUR,
 				   USB_TYPE_CLASS | USB_RECIP_INTERFACE | USB_DIR_IN,
 				   UAC2_CS_CONTROL_SAM_FREQ << 8,
 				   snd_usb_ctrl_intf(chip) | (clock << 8),
-				   &data, sizeof(rate))) < 0) {
+				   &data, sizeof(rate));
+	if (err < 0) {
 		snd_printk(KERN_WARNING "%d:%d:%d: cannot get freq (v2): err %d\n",
 			   dev->devnum, iface, fmt->altsetting, err);
 		return err;
 	}
 
 	crate = le32_to_cpu(data);
-	if (crate != rate)
+	if (crate != 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, crate);
+			return -ENXIO;
+		}
 		snd_printd(KERN_WARNING "current rate %d is different from the runtime rate %d\n", crate, rate);
+	}
 
 	return 0;
 }
-- 
1.8.1.5