[media] uvcvideo: uvc_scan_fallback() for webcams with broken chain

Some devices have invalid baSourceID references, causing uvc_scan_chain()
to fail, but if we just take the entities we can find and put them
together in the most sensible chain we can think of, turns out they do
work anyway. Note: This heuristic assumes there is a single chain.

At the time of writing, devices known to have such a broken chain are
  - Acer Integrated Camera (5986:055a)
  - Realtek rtl157a7 (0bda:57a7)

Signed-off-by: Henrik Ingo <henrik.ingo@avoinelama.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/usb/uvc/uvc_driver.c

@@ -1595,6 +1595,114 @@ static const char *uvc_print_chain(struct uvc_video_chain *chain)
 	return buffer;
 }
 
+static struct uvc_video_chain *uvc_alloc_chain(struct uvc_device *dev)
+{
+	struct uvc_video_chain *chain;
+
+	chain = kzalloc(sizeof(*chain), GFP_KERNEL);
+	if (chain == NULL)
+		return NULL;
+
+	INIT_LIST_HEAD(&chain->entities);
+	mutex_init(&chain->ctrl_mutex);
+	chain->dev = dev;
+	v4l2_prio_init(&chain->prio);
+
+	return chain;
+}
+
+/*
+ * Fallback heuristic for devices that don't connect units and terminals in a
+ * valid chain.
+ *
+ * Some devices have invalid baSourceID references, causing uvc_scan_chain()
+ * to fail, but if we just take the entities we can find and put them together
+ * in the most sensible chain we can think of, turns out they do work anyway.
+ * Note: This heuristic assumes there is a single chain.
+ *
+ * At the time of writing, devices known to have such a broken chain are
+ *  - Acer Integrated Camera (5986:055a)
+ *  - Realtek rtl157a7 (0bda:57a7)
+ */
+static int uvc_scan_fallback(struct uvc_device *dev)
+{
+	struct uvc_video_chain *chain;
+	struct uvc_entity *iterm = NULL;
+	struct uvc_entity *oterm = NULL;
+	struct uvc_entity *entity;
+	struct uvc_entity *prev;
+
+	/*
+	 * Start by locating the input and output terminals. We only support
+	 * devices with exactly one of each for now.
+	 */
+	list_for_each_entry(entity, &dev->entities, list) {
+		if (UVC_ENTITY_IS_ITERM(entity)) {
+			if (iterm)
+				return -EINVAL;
+			iterm = entity;
+		}
+
+		if (UVC_ENTITY_IS_OTERM(entity)) {
+			if (oterm)
+				return -EINVAL;
+			oterm = entity;
+		}
+	}
+
+	if (iterm == NULL || oterm == NULL)
+		return -EINVAL;
+
+	/* Allocate the chain and fill it. */
+	chain = uvc_alloc_chain(dev);
+	if (chain == NULL)
+		return -ENOMEM;
+
+	if (uvc_scan_chain_entity(chain, oterm) < 0)
+		goto error;
+
+	prev = oterm;
+
+	/*
+	 * Add all Processing and Extension Units with two pads. The order
+	 * doesn't matter much, use reverse list traversal to connect units in
+	 * UVC descriptor order as we build the chain from output to input. This
+	 * leads to units appearing in the order meant by the manufacturer for
+	 * the cameras known to require this heuristic.
+	 */
+	list_for_each_entry_reverse(entity, &dev->entities, list) {
+		if (entity->type != UVC_VC_PROCESSING_UNIT &&
+		    entity->type != UVC_VC_EXTENSION_UNIT)
+			continue;
+
+		if (entity->num_pads != 2)
+			continue;
+
+		if (uvc_scan_chain_entity(chain, entity) < 0)
+			goto error;
+
+		prev->baSourceID[0] = entity->id;
+		prev = entity;
+	}
+
+	if (uvc_scan_chain_entity(chain, iterm) < 0)
+		goto error;
+
+	prev->baSourceID[0] = iterm->id;
+
+	list_add_tail(&chain->list, &dev->chains);
+
+	uvc_trace(UVC_TRACE_PROBE,
+		  "Found a video chain by fallback heuristic (%s).\n",
+		  uvc_print_chain(chain));
+
+	return 0;
+
+error:
+	kfree(chain);
+	return -EINVAL;
+}
+
 /*
  * Scan the device for video chains and register video devices.
  *
@@ -1617,15 +1725,10 @@ static int uvc_scan_device(struct uvc_device *dev)
 		if (term->chain.next || term->chain.prev)
 			continue;
 
-		chain = kzalloc(sizeof(*chain), GFP_KERNEL);
+		chain = uvc_alloc_chain(dev);
 		if (chain == NULL)
 			return -ENOMEM;
 
-		INIT_LIST_HEAD(&chain->entities);
-		mutex_init(&chain->ctrl_mutex);
-		chain->dev = dev;
-		v4l2_prio_init(&chain->prio);
-
 		term->flags |= UVC_ENTITY_FLAG_DEFAULT;
 
 		if (uvc_scan_chain(chain, term) < 0) {
@@ -1639,6 +1742,9 @@ static int uvc_scan_device(struct uvc_device *dev)
 		list_add_tail(&chain->list, &dev->chains);
 	}
 
+	if (list_empty(&dev->chains))
+		uvc_scan_fallback(dev);
+
 	if (list_empty(&dev->chains)) {
 		uvc_printk(KERN_INFO, "No valid video chain found.\n");
 		return -1;