drm/amdgpu: Fix error handling in amdgpu_flip_work_func.

The patch e1d09dc0ccc6: "drm/amdgpu: Don't hang in
amdgpu_flip_work_func on disabled crtc." from Feb 19, 2016, leads to
the following static checker warning, as reported by Dan Carpenter in
https://lists.freedesktop.org/archives/dri-devel/2016-February/101987.html

drivers/gpu/drm/amd/amdgpu/amdgpu_display.c:127 amdgpu_flip_work_func()	warn: should this be 'repcnt == -1'
drivers/gpu/drm/amd/amdgpu/amdgpu_display.c:136 amdgpu_flip_work_func() error: double unlock 'spin_lock:&crtc->dev->event_lock'
drivers/gpu/drm/amd/amdgpu/amdgpu_display.c:136 amdgpu_flip_work_func() error: double unlock 'irqsave:flags'

This patch fixes both reported problems:

Change post-decrement of repcnt to pre-decrement, so
it can't underflow anymore, but still performs up to
three repetitions - three is the maximum one could
expect in practice.

Move the spin_unlock_irqrestore to where it actually
belongs.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: <stable@vger.kernel.org> # 4.4+
Cc: Michel Dänzer <michel.daenzer@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_display.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_display.c
index 8297bc319369..1846d65b7285 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_display.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_display.c
@@ -96,7 +96,7 @@ static void amdgpu_flip_work_func(struct work_struct *__work)
          * In practice this won't execute very often unless on very fast
          * machines because the time window for this to happen is very small.
          */
-        while (amdgpuCrtc->enabled && repcnt--) {
+        while (amdgpuCrtc->enabled && --repcnt) {
                 /* GET_DISTANCE_TO_VBLANKSTART returns distance to real vblank
                  * start in hpos, and to the "fudged earlier" vblank start in
                  * vpos.
@@ -112,13 +112,13 @@ static void amdgpu_flip_work_func(struct work_struct *__work)
                         break;
 
                 /* Sleep at least until estimated real start of hw vblank */
-                spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
                 min_udelay = (-hpos + 1) * max(vblank->linedur_ns / 1000, 5);
                 if (min_udelay > vblank->framedur_ns / 2000) {
                         /* Don't wait ridiculously long - something is wrong */
                         repcnt = 0;
                         break;
                 }
+                spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
                 usleep_range(min_udelay, 2 * min_udelay);
                 spin_lock_irqsave(&crtc->dev->event_lock, flags);
         };