[Intel-gfx] [PATCH] drm: use seqlock for vblank time/count
Ville Syrjälä
ville.syrjala at linux.intel.com
Tue May 24 12:20:54 UTC 2016
On Tue, May 10, 2016 at 03:21:28PM +0100, Matthew Auld wrote:
> 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 at ffwll.ch>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
LGTM
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.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
--
Ville Syrjälä
Intel OTC
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