drm/nouveau: fix a race condition in nouveau_dma_wait()

Can be triggered easily on certain cards (NV46 and NV50 of mine) by
running "dmesg", the DRM's channel will lockup.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_dma.c

@@ -126,47 +126,52 @@ OUT_RINGp(struct nouveau_channel *chan, const void *data, unsigned nr_dwords)
 	chan->dma.cur += nr_dwords;
 }
 
-static inline bool
-READ_GET(struct nouveau_channel *chan, uint32_t *get)
+/* Fetch and adjust GPU GET pointer
+ *
+ * Returns:
+ *  value >= 0, the adjusted GET pointer
+ *  -EINVAL if GET pointer currently outside main push buffer
+ *  -EBUSY if timeout exceeded
+ */
+static inline int
+READ_GET(struct nouveau_channel *chan, uint32_t *prev_get, uint32_t *timeout)
 {
 	uint32_t val;
 
 	val = nvchan_rd32(chan, chan->user_get);
-	if (val < chan->pushbuf_base ||
-	    val > chan->pushbuf_base + (chan->dma.max << 2)) {
-		/* meaningless to dma_wait() except to know whether the
-		 * GPU has stalled or not
-		 */
-		*get = val;
-		return false;
+
+	/* reset counter as long as GET is still advancing, this is
+	 * to avoid misdetecting a GPU lockup if the GPU happens to
+	 * just be processing an operation that takes a long time
+	 */
+	if (val != *prev_get) {
+		*prev_get = val;
+		*timeout = 0;
+	}
+
+	if ((++*timeout & 0xff) == 0) {
+		DRM_UDELAY(1);
+		if (*timeout > 100000)
+			return -EBUSY;
 	}
 
-	*get = (val - chan->pushbuf_base) >> 2;
-	return true;
+	if (val < chan->pushbuf_base ||
+	    val > chan->pushbuf_base + (chan->dma.max << 2))
+		return -EINVAL;
+
+	return (val - chan->pushbuf_base) >> 2;
 }
 
 int
 nouveau_dma_wait(struct nouveau_channel *chan, int size)
 {
-	uint32_t get, prev_get = 0, cnt = 0;
-	bool get_valid;
+	uint32_t prev_get = 0, cnt = 0;
+	int get;
 
 	while (chan->dma.free < size) {
-		/* reset counter as long as GET is still advancing, this is
-		 * to avoid misdetecting a GPU lockup if the GPU happens to
-		 * just be processing an operation that takes a long time
-		 */
-		get_valid = READ_GET(chan, &get);
-		if (get != prev_get) {
-			prev_get = get;
-			cnt = 0;
-		}
-
-		if ((++cnt & 0xff) == 0) {
-			DRM_UDELAY(1);
-			if (cnt > 100000)
-				return -EBUSY;
-		}
+		get = READ_GET(chan, &prev_get, &cnt);
+		if (unlikely(get == -EBUSY))
+			return -EBUSY;
 
 		/* loop until we have a usable GET pointer.  the value
 		 * we read from the GPU may be outside the main ring if
@@ -177,7 +182,7 @@ nouveau_dma_wait(struct nouveau_channel *chan, int size)
 		 * from the SKIPS area, so the code below doesn't have to deal
 		 * with some fun corner cases.
 		 */
-		if (!get_valid || get < NOUVEAU_DMA_SKIPS)
+		if (unlikely(get == -EINVAL) || get < NOUVEAU_DMA_SKIPS)
 			continue;
 
 		if (get <= chan->dma.cur) {
@@ -203,6 +208,19 @@ nouveau_dma_wait(struct nouveau_channel *chan, int size)
 			 * after processing the currently pending commands.
 			 */
 			OUT_RING(chan, chan->pushbuf_base | 0x20000000);
+
+			/* wait for GET to depart from the skips area.
+			 * prevents writing GET==PUT and causing a race
+			 * condition that causes us to think the GPU is
+			 * idle when it's not.
+			 */
+			do {
+				get = READ_GET(chan, &prev_get, &cnt);
+				if (unlikely(get == -EBUSY))
+					return -EBUSY;
+				if (unlikely(get == -EINVAL))
+					continue;
+			} while (get <= NOUVEAU_DMA_SKIPS);
 			WRITE_PUT(NOUVEAU_DMA_SKIPS);
 
 			/* we're now submitting commands at the start of