[PATCH] drm: use seqlock for vblank time/count

[Matthew Auld <matthew.auld@intel.com>]

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: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
---
 drivers/gpu/drm/drm_irq.c | 90 +++++++----------------------------------------
 include/drm/drmP.h        | 14 +++-----
 2 files changed, 17 insertions(+), 87 deletions(-)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 3c1a6f1..0e95100 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -42,10 +42,6 @@
 #include <linux/vgaarb.h>
 #include <linux/export.h>
 
-/* Access macro for slots in vblank timestamp ringbuffer. */
-#define vblanktimestamp(dev, pipe, count) \
-	((dev)->vblank[pipe].time[(count) % DRM_VBLANKTIME_RBSIZE])
-
 /* Retry timestamp calculation up to 3 times to satisfy
  * drm_timestamp_precision before giving up.
  */
@@ -82,29 +78,15 @@ static void store_vblank(struct drm_device *dev, unsigned int pipe,
 			 struct timeval *t_vblank, u32 last)
 {
 	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
-	u32 tslot;
 
 	assert_spin_locked(&dev->vblank_time_lock);
 
 	vblank->last = last;
 
-	/* All writers hold the spinlock, but readers are serialized by
-	 * the latching of vblank->count below.
-	 */
-	tslot = vblank->count + vblank_count_inc;
-	vblanktimestamp(dev, pipe, tslot) = *t_vblank;
-
-	/*
-	 * vblank timestamp updates are protected on the write side with
-	 * vblank_time_lock, but on the read side done locklessly using a
-	 * sequence-lock on the vblank counter. Ensure correct ordering using
-	 * memory barrriers. We need the barrier both before and also after the
-	 * counter update to synchronize with the next timestamp write.
-	 * The read-side barriers for this are in drm_vblank_count_and_time.
-	 */
-	smp_wmb();
+	write_seqlock(&vblank->seqlock);
+	vblank->time = *t_vblank;
 	vblank->count += vblank_count_inc;
-	smp_wmb();
+	write_sequnlock(&vblank->seqlock);
 }
 
 /**
@@ -205,7 +187,7 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
 		const struct timeval *t_old;
 		u64 diff_ns;
 
-		t_old = &vblanktimestamp(dev, pipe, vblank->count);
+		t_old = &vblank->time;
 		diff_ns = timeval_to_ns(&t_vblank) - timeval_to_ns(t_old);
 
 		/*
@@ -239,49 +221,6 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
 		diff = 1;
 	}
 
-	/*
-	 * FIMXE: Need to replace this hack with proper seqlocks.
-	 *
-	 * Restrict the bump of the software vblank counter to a safe maximum
-	 * value of +1 whenever there is the possibility that concurrent readers
-	 * of vblank timestamps could be active at the moment, as the current
-	 * implementation of the timestamp caching and updating is not safe
-	 * against concurrent readers for calls to store_vblank() with a bump
-	 * of anything but +1. A bump != 1 would very likely return corrupted
-	 * timestamps to userspace, because the same slot in the cache could
-	 * be concurrently written by store_vblank() and read by one of those
-	 * readers without the read-retry logic detecting the collision.
-	 *
-	 * Concurrent readers can exist when we are called from the
-	 * drm_vblank_off() or drm_vblank_on() functions and other non-vblank-
-	 * irq callers. However, all those calls to us are happening with the
-	 * vbl_lock locked to prevent drm_vblank_get(), so the vblank refcount
-	 * can't increase while we are executing. Therefore a zero refcount at
-	 * this point is safe for arbitrary counter bumps if we are called
-	 * outside vblank irq, a non-zero count is not 100% safe. Unfortunately
-	 * we must also accept a refcount of 1, as whenever we are called from
-	 * drm_vblank_get() -> drm_vblank_enable() the refcount will be 1 and
-	 * we must let that one pass through in order to not lose vblank counts
-	 * during vblank irq off - which would completely defeat the whole
-	 * point of this routine.
-	 *
-	 * Whenever we are called from vblank irq, we have to assume concurrent
-	 * readers exist or can show up any time during our execution, even if
-	 * the refcount is currently zero, as vblank irqs are usually only
-	 * enabled due to the presence of readers, and because when we are called
-	 * from vblank irq we can't hold the vbl_lock to protect us from sudden
-	 * bumps in vblank refcount. Therefore also restrict bumps to +1 when
-	 * called from vblank irq.
-	 */
-	if ((diff > 1) && (atomic_read(&vblank->refcount) > 1 ||
-	    (flags & DRM_CALLED_FROM_VBLIRQ))) {
-		DRM_DEBUG_VBL("clamping vblank bump to 1 on crtc %u: diffr=%u "
-			      "refcount %u, vblirq %u\n", pipe, diff,
-			      atomic_read(&vblank->refcount),
-			      (flags & DRM_CALLED_FROM_VBLIRQ) != 0);
-		diff = 1;
-	}
-
 	DRM_DEBUG_VBL("updating vblank count on crtc %u:"
 		      " current=%u, diff=%u, hw=%u hw_last=%u\n",
 		      pipe, vblank->count, diff, cur_vblank, vblank->last);
@@ -420,6 +359,7 @@ int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs)
 		init_waitqueue_head(&vblank->queue);
 		setup_timer(&vblank->disable_timer, vblank_disable_fn,
 			    (unsigned long)vblank);
+		seqlock_init(&vblank->seqlock);
 	}
 
 	DRM_INFO("Supports vblank timestamp caching Rev 2 (21.10.2013).\n");
@@ -991,25 +931,19 @@ u32 drm_vblank_count_and_time(struct drm_device *dev, unsigned int pipe,
 			      struct timeval *vblanktime)
 {
 	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
-	int count = DRM_TIMESTAMP_MAXRETRIES;
-	u32 cur_vblank;
+	u32 vblank_count;
+	unsigned int seq;
 
 	if (WARN_ON(pipe >= dev->num_crtcs))
 		return 0;
 
-	/*
-	 * Vblank timestamps are read lockless. To ensure consistency the vblank
-	 * counter is rechecked and ordering is ensured using memory barriers.
-	 * This works like a seqlock. The write-side barriers are in store_vblank.
-	 */
 	do {
-		cur_vblank = vblank->count;
-		smp_rmb();
-		*vblanktime = vblanktimestamp(dev, pipe, cur_vblank);
-		smp_rmb();
-	} while (cur_vblank != vblank->count && --count > 0);
+		seq = read_seqbegin(&vblank->seqlock);
+		vblank_count = vblank->count;
+		*vblanktime = vblank->time;
+	} while (read_seqretry(&vblank->seqlock, seq));
 
-	return cur_vblank;
+	return vblank_count;
 }
 EXPORT_SYMBOL(drm_vblank_count_and_time);
 
diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 360b2a7..9f33090 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 */
-- 
2.4.11

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