[PATCH] drm/i915: use seqlocks for vlbank time/count
Matthew Auld
matthew.auld at intel.com
Thu May 5 15:13:26 UTC 2016
This patch aims to replace the roll-your-own seqlock implementation with
actual seqlocks. This removes the timestamp ringbuffer in favour of
single timestamp/count pair protected by a seqlock. In turn this means
we can now increment the vlbank count by anything.
Signed-off-by: Matthew Auld <matthew.auld at intel.com>
---
drivers/gpu/drm/drm_irq.c | 86 +++++++----------------------------------------
include/drm/drmP.h | 12 +++----
2 files changed, 17 insertions(+), 81 deletions(-)
diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 3c1a6f1..ea5c88c 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -82,29 +82,15 @@ 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->seq_lock);
+ vblank->time = *t_vblank;
vblank->count += vblank_count_inc;
- smp_wmb();
+ write_sequnlock(&vblank->seq_lock);
}
/**
@@ -205,7 +191,7 @@ 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 +225,6 @@ 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 +363,7 @@ 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->seq_lock);
}
DRM_INFO("Supports vblank timestamp caching Rev 2 (21.10.2013).\n");
@@ -991,25 +935,19 @@ 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->seq_lock);
+ vblank_count = vblank->count;
+ *vblanktime = vblank->time;
+ } while (read_seqretry(&vblank->seq_lock, 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..2d9b10e 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -56,6 +56,7 @@
#include <linux/types.h>
#include <linux/vmalloc.h>
#include <linux/workqueue.h>
+#include <linux/seqlock.h>
#include <asm/mman.h>
#include <asm/pgalloc.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 +721,12 @@ struct drm_vblank_crtc {
wait_queue_head_t queue; /**< VBLANK wait queue */
struct timer_list disable_timer; /* delayed disable timer */
+ seqlock_t seq_lock;
+
/* 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];
+ /* vblank timestamp, protected by dev->vblank_time_lock for writes */
+ struct timeval time;
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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