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 --- 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 */