Patch "drm/i915: Fixup the free space logic in ring_prepare" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

drm/i915: Fixup the free space logic in ring_prepare

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-i915-fixup-the-free-space-logic-in-ring_prepare.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Akash Goel <[email protected]>
Date: Fri, 11 Mar 2016 14:56:42 +0530
Subject: drm/i915: Fixup the free space logic in ring_prepare

From: Akash Goel <[email protected]>

commit d43f3ebf12f59c57782ec652da65ef61c2662b40 upstream.

Currently for the case where there is enough space at the end of Ring buffer for accommodating only the base request, the wrapround is done immediately and as a result the base request gets added at the start of Ring buffer. But there may not be enough free space at the beginning to accommodate the base request, as before the wraparound, the wait was effectively done for the reserved_size free space from the start of Ring buffer. In such a case there is a potential of Ring buffer overflow, the instructions at the head of Ring (ACTHD) can get overwritten.

Since the base request can fit in the remaining space, there is no need to wraparound immediately. The wraparound will anyway happen later when the reserved part starts getting used.

---
 drivers/gpu/drm/i915/intel_lrc.c        |    6 +++---
 drivers/gpu/drm/i915/intel_ringbuffer.c |    6 +++---
 2 files changed, 6 insertions(+), 6 deletions(-)

@@ -776,11 +776,11 @@ static int logical_ring_prepare(struct d
 		if (unlikely(total_bytes > remain_usable)) {
 			/*
 			 * The base request will fit but the reserved space
-			 * falls off the end. So only need to to wait for the
-			 * reserved size after flushing out the remainder.
+			 * falls off the end. So don't need an immediate wrap
+			 * and only need to effectively wait for the reserved
+			 * size space from the start of ringbuffer.
 			 */
 			wait_bytes = remain_actual + ringbuf->reserved_size;
-			need_wrap = true;
 		} else if (total_bytes > ringbuf->space) {
 			/* No wrapping required, just waiting. */
 			wait_bytes = total_bytes;

@@ -2357,11 +2357,11 @@ static int __intel_ring_prepare(struct i
 		if (unlikely(total_bytes > remain_usable)) {
 			/*
 			 * The base request will fit but the reserved space
-			 * falls off the end. So only need to to wait for the
-			 * reserved size after flushing out the remainder.
+			 * falls off the end. So don't need an immediate wrap
+			 * and only need to effectively wait for the reserved
+			 * size space from the start of ringbuffer.
 			 */
 			wait_bytes = remain_actual + ringbuf->reserved_size;
-			need_wrap = true;
 		} else if (total_bytes > ringbuf->space) {
 			/* No wrapping required, just waiting. */
 			wait_bytes = total_bytes;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/drm-i915-fixup-the-free-space-logic-in-ring_prepare.patch