Behringer UMC202HD issues and a partial solution

[Geraldo Nascimento]

On Sat, Jan 15, 2022 at 12:29:18PM +0530, Nandakumar Edamana wrote:
> Could compile and reload the module successfully, but the stuttering is 
> still there.

Hi Nandakumar,

OK, my mistake, scratch that patch.

The codepath that activates Implicit Feedback when implicit_fb=1 is
passed to the module as an option is the same that should activate when
the quirk is in place.

Like I said, it's very strange that your device skips implicit feedback
when the quirk is in place, but can pick implicit feedback when
implicit_fb=1 is passed to the module as an option.

I've prepared some printk's hooked up to every codepath I can think of
that will skip implicit feedback. Please test and share your results,
always with dyndbg on.

--- implicit.c	2022-01-14 16:41:53.946606013 -0300
+++ implicit.c	2022-01-15 21:45:15.108655890 -0300
@@ -90,8 +90,10 @@ static int add_implicit_fb_sync_ep(struc
 
 	if (!alts) {
 		iface = usb_ifnum_to_if(chip->dev, ifnum);
-		if (!iface || iface->num_altsetting < 2)
+		if (!iface || iface->num_altsetting < 2) {
+			printk(KERN_WARNING "implicit: add_implicit_fb_sync_ep(): not a usb_host_interface or less than 2 altsettings");
 			return 0;
+		}
 		alts = &iface->altsetting[1];
 	}
 
@@ -242,17 +244,23 @@ static int __add_generic_implicit_fb(str
 	struct usb_endpoint_descriptor *epd;
 
 	alts = snd_usb_get_host_interface(chip, iface, altset);
-	if (!alts)
+	if (!alts) {
+		printk(KERN_WARNING "implicit: __add_generic_implicit_fb(): not a usb_host_interface");
 		return 0;
+	}
 
 	if ((alts->desc.bInterfaceClass != USB_CLASS_VENDOR_SPEC &&
 	     alts->desc.bInterfaceClass != USB_CLASS_AUDIO) ||
-	    alts->desc.bNumEndpoints < 1)
+	    alts->desc.bNumEndpoints < 1) {
+		printk(KERN_WARNING "implicit: __add_generic_implicit_fb(): no endpoints or wrong Interface Class");
 		return 0;
+	}
 	epd = get_endpoint(alts, 0);
 	if (!usb_endpoint_is_isoc_in(epd) ||
-	    (epd->bmAttributes & USB_ENDPOINT_SYNCTYPE) != USB_ENDPOINT_SYNC_ASYNC)
+	    (epd->bmAttributes & USB_ENDPOINT_SYNCTYPE) != USB_ENDPOINT_SYNC_ASYNC) {
+		printk(KERN_WARNING "implicit: __add_generic_implicit_fb(): not Isochronous IN or not Asynchronous");
 		return 0;
+	}
 	return add_implicit_fb_sync_ep(chip, fmt, epd->bEndpointAddress, 0,
 				       iface, alts);
 }
@@ -262,8 +270,10 @@ static int add_generic_implicit_fb(struc
 				   struct audioformat *fmt,
 				   struct usb_host_interface *alts)
 {
-	if ((fmt->ep_attr & USB_ENDPOINT_SYNCTYPE) != USB_ENDPOINT_SYNC_ASYNC)
+	if ((fmt->ep_attr & USB_ENDPOINT_SYNCTYPE) != USB_ENDPOINT_SYNC_ASYNC) {
+		printk(KERN_WARNING "implicit: add_generic_implicit_fb(): not Asynchronous");
 		return 0;
+	}
 
 	if (__add_generic_implicit_fb(chip, fmt,
 				      alts->desc.bInterfaceNumber + 1,
@@ -302,8 +312,10 @@ static int audioformat_implicit_fb_quirk
 	if (p) {
 		switch (p->type) {
 		case IMPLICIT_FB_GENERIC:
+			printk(KERN_WARNING "implicit: audioformat_implicit_fb_quirk(): matched GENERIC inside playback_implicit_fb_quirks[]");
 			return add_generic_implicit_fb(chip, fmt, alts);
 		case IMPLICIT_FB_NONE:
+			printk(KERN_WARNING "implicit: audioformat_implicit_fb_quirk(): matched SKIP_DEV inside playback_implicit_fb_quirks[]");
 			return 0; /* No quirk */
 		case IMPLICIT_FB_FIXED:
 			return add_implicit_fb_sync_ep(chip, fmt, p->ep_num, 0,
@@ -311,6 +323,10 @@ static int audioformat_implicit_fb_quirk
 		}
 	}
 
+	else {
+		printk(KERN_WARNING "implicit: audioformat_implicit_fb_quirk(): no matches found inside playback_implicit_fb_quirks[]");
+	}
+
 	/* Special handling for devices with capture quirks */
 	p = find_implicit_fb_entry(chip, capture_implicit_fb_quirks, alts);
 	if (p) {
@@ -354,6 +370,7 @@ static int audioformat_implicit_fb_quirk
 		return add_generic_implicit_fb(chip, fmt, alts);
 
 	/* No quirk */
+	printk(KERN_WARNING "implicit: audioformat_implicit_fb_quirk(): returning 0 because no quirk was found");
 	return 0;
 }