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.
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 | 111 +++++++++------------------------------------- include/drm/drmP.h | 14 ++---- 2 files changed, 25 insertions(+), 100 deletions(-)
diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c index 3c1a6f1..bfc6a8d 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 +78,13 @@ 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); + assert_spin_locked(&dev->vblank_seqlock.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(); + vblank->time = *t_vblank; vblank->count += vblank_count_inc; - smp_wmb(); }
/** @@ -127,7 +107,7 @@ static void drm_reset_vblank_timestamp(struct drm_device *dev, unsigned int pipe struct timeval t_vblank; int count = DRM_TIMESTAMP_MAXRETRIES;
- spin_lock(&dev->vblank_time_lock); + write_seqlock(&dev->vblank_seqlock);
/* * sample the current counter to avoid random jumps @@ -152,7 +132,7 @@ static void drm_reset_vblank_timestamp(struct drm_device *dev, unsigned int pipe */ store_vblank(dev, pipe, 1, &t_vblank, cur_vblank);
- spin_unlock(&dev->vblank_time_lock); + write_sequnlock(&dev->vblank_seqlock); }
/** @@ -205,7 +185,7 @@ 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 +219,6 @@ 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); @@ -318,7 +255,7 @@ static void vblank_disable_and_save(struct drm_device *dev, unsigned int pipe) * so no updates of timestamps or count can happen after we've * disabled. Needed to prevent races in case of delayed irq's. */ - spin_lock_irqsave(&dev->vblank_time_lock, irqflags); + write_seqlock_irqsave(&dev->vblank_seqlock, irqflags);
/* * Only disable vblank interrupts if they're enabled. This avoids @@ -338,7 +275,7 @@ static void vblank_disable_and_save(struct drm_device *dev, unsigned int pipe) */ drm_update_vblank_count(dev, pipe, 0);
- spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags); + write_sequnlock_irqrestore(&dev->vblank_seqlock, irqflags); }
static void vblank_disable_fn(unsigned long arg) @@ -404,7 +341,7 @@ int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs) unsigned int i;
spin_lock_init(&dev->vbl_lock); - spin_lock_init(&dev->vblank_time_lock); + seqlock_init(&dev->vblank_seqlock);
dev->num_crtcs = num_crtcs;
@@ -991,25 +928,19 @@ 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(&dev->vblank_seqlock); + vblank_count = vblank->count; + *vblanktime = vblank->time; + } while (read_seqretry(&dev->vblank_seqlock, seq));
- return cur_vblank; + return vblank_count; } EXPORT_SYMBOL(drm_vblank_count_and_time);
@@ -1160,11 +1091,11 @@ static int drm_vblank_enable(struct drm_device *dev, unsigned int pipe)
assert_spin_locked(&dev->vbl_lock);
- spin_lock(&dev->vblank_time_lock); + write_seqlock(&dev->vblank_seqlock);
if (!vblank->enabled) { /* - * Enable vblank irqs under vblank_time_lock protection. + * Enable vblank irqs under vblank_seqlock protection. * All vblank count & timestamp updates are held off * until we are done reinitializing master counter and * timestamps. Filtercode in drm_handle_vblank() will @@ -1180,7 +1111,7 @@ static int drm_vblank_enable(struct drm_device *dev, unsigned int pipe) } }
- spin_unlock(&dev->vblank_time_lock); + write_sequnlock(&dev->vblank_seqlock);
return ret; } @@ -1880,18 +1811,18 @@ bool drm_handle_vblank(struct drm_device *dev, unsigned int pipe) * vblank enable/disable, as this would cause inconsistent * or corrupted timestamps and vblank counts. */ - spin_lock(&dev->vblank_time_lock); + write_seqlock(&dev->vblank_seqlock);
/* Vblank irq handling disabled. Nothing to do. */ if (!vblank->enabled) { - spin_unlock(&dev->vblank_time_lock); + write_sequnlock(&dev->vblank_seqlock); spin_unlock_irqrestore(&dev->event_lock, irqflags); return false; }
drm_update_vblank_count(dev, pipe, DRM_CALLED_FROM_VBLIRQ);
- spin_unlock(&dev->vblank_time_lock); + write_sequnlock(&dev->vblank_seqlock);
wake_up(&vblank->queue); drm_handle_vblank_events(dev, pipe); diff --git a/include/drm/drmP.h b/include/drm/drmP.h index 360b2a7..8bee424 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 +721,8 @@ 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]; + u32 count; /* vblank counter, protected by dev->vblank_seqlock */ + struct timeval time; /* vblank timestamp, protected by dev->vblank_seqlock */
atomic_t refcount; /* number of users of vblank interruptsper crtc */ u32 last; /* protected by dev->vbl_lock, used */ @@ -835,7 +829,7 @@ struct drm_device { /* array of size num_crtcs */ struct drm_vblank_crtc *vblank;
- spinlock_t vblank_time_lock; /**< Protects vblank count and time updates during vblank enable/disable */ + seqlock_t vblank_seqlock; /**< Protects vblank count and time updates during vblank enable/disable */ spinlock_t vbl_lock;
u32 max_vblank_count; /**< size of vblank counter register */