[PATCH] USB: Always select config with the highest supported UAC version

[Alexander Tsoy <alexander@xxxxxxx>]

Config with the highest supported UAC version is always preferable because
it usually provides more features.

Main motivation for this change is to improve support for several UAC2
devices which have UAC1 config as the first one for compatibility reasons.
UAC2 mode provides a wider range of supported sampling rates on these
devices. Also when UAC4 support is implemented, it will need just one
additional case line without changing the logic.

Signed-off-by: Alexander Tsoy <alexander@xxxxxxx>
---
 drivers/usb/core/generic.c | 39 +++++++++++++++++++++++++-------------
 1 file changed, 26 insertions(+), 13 deletions(-)

diff --git a/drivers/usb/core/generic.c b/drivers/usb/core/generic.c
index b134bff5c3fe..8aeb180e1cf9 100644
--- a/drivers/usb/core/generic.c
+++ b/drivers/usb/core/generic.c
@@ -48,9 +48,16 @@ static bool is_audio(struct usb_interface_descriptor *desc)
 	return desc->bInterfaceClass == USB_CLASS_AUDIO;
 }
 
-static bool is_uac3_config(struct usb_interface_descriptor *desc)
+static bool is_supported_uac(struct usb_interface_descriptor *desc)
 {
-	return desc->bInterfaceProtocol == UAC_VERSION_3;
+	switch(desc->bInterfaceProtocol) {
+	case UAC_VERSION_1:
+	case UAC_VERSION_2:
+	case UAC_VERSION_3:
+		return true;
+	default:
+		return false;
+	}
 }
 
 int usb_choose_configuration(struct usb_device *udev)
@@ -135,22 +142,28 @@ int usb_choose_configuration(struct usb_device *udev)
 		}
 
 		/*
-		 * Select first configuration as default for audio so that
-		 * devices that don't comply with UAC3 protocol are supported.
-		 * But, still iterate through other configurations and
-		 * select UAC3 compliant config if present.
+		 * Iterate through audio configurations and select the first of
+		 * the highest supported UAC versions. Select the first config
+		 * if no supported UAC configs are found.
 		 */
 		if (desc && is_audio(desc)) {
-			/* Always prefer the first found UAC3 config */
-			if (is_uac3_config(desc)) {
-				best = c;
-				break;
-			}
+			struct usb_interface_descriptor	*best_desc = NULL;
+
+			if (best)
+				best_desc = &best->intf_cache[0]->altsetting->desc;
 
-			/* If there is no UAC3 config, prefer the first config */
-			else if (i == 0)
+			if (i == 0)
 				best = c;
 
+			/* Assume that bInterfaceProtocol value is always
+			 * growing when UAC versions are incremented, so that
+			 * the direct comparison is possible. */
+			else if (is_supported_uac(desc) && best_desc &&
+				 (!is_supported_uac(best_desc) ||
+				  (desc->bInterfaceProtocol >
+				   best_desc->bInterfaceProtocol)))
+					best = c;
+
 			/* Unconditional continue, because the rest of the code
 			 * in the loop is irrelevant for audio devices, and
 			 * because it can reassign best, which for audio devices
-- 
2.43.0