drm: use seqlock for vblank time/count

This patch aims to replace the roll-your-own seqlock implementation with
full-blown seqlock'. We also remove the timestamp ring-buffer in favour
of single timestamp/count pair protected by a seqlock. In turn this
means we can now increment the vblank freely without the need for
clamping.

v2:
  - reduce the scope of the seqlock, keeping vblank_time_lock
  - make the seqlock per vblank_crtc, so multiple readers aren't blocked by
    the writer

Cc: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462890088-18194-1-git-send-email-matthew.auld@intel.com

1 files changed, 5 insertions, 9 deletions

diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 84f1a8eefbdb..af8b71b2e5bd 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -52,6 +52,7 @@
 #include <linux/poll.h>
 #include <linux/ratelimit.h>
 #include <linux/sched.h>
+#include <linux/seqlock.h>
 #include <linux/slab.h>
 #include <linux/types.h>
 #include <linux/vmalloc.h>
@@ -392,11 +393,6 @@ struct drm_master {
 	void *driver_priv;
 };
 
-/* Size of ringbuffer for vblank timestamps. Just double-buffer
- * in initial implementation.
- */
-#define DRM_VBLANKTIME_RBSIZE 2
-
 /* Flags and return codes for get_vblank_timestamp() driver function. */
 #define DRM_CALLED_FROM_VBLIRQ 1
 #define DRM_VBLANKTIME_SCANOUTPOS_METHOD (1 << 0)
@@ -725,10 +721,10 @@ struct drm_vblank_crtc {
 	wait_queue_head_t queue;	/**< VBLANK wait queue */
 	struct timer_list disable_timer;		/* delayed disable timer */
 
-	/* vblank counter, protected by dev->vblank_time_lock for writes */
-	u32 count;
-	/* vblank timestamps, protected by dev->vblank_time_lock for writes */
-	struct timeval time[DRM_VBLANKTIME_RBSIZE];
+	seqlock_t seqlock;		/* protects vblank count and time */
+
+	u32 count;			/* vblank counter */
+	struct timeval time;		/* vblank timestamp */
 
 	atomic_t refcount;		/* number of users of vblank interruptsper crtc */
 	u32 last;			/* protected by dev->vbl_lock, used */