[Intel-gfx] [PATCH] drm: use seqlocks for vblank time/count
Mario Kleiner
mario.kleiner.de at gmail.com
Wed May 18 13:54:39 UTC 2016
On 05/09/2016 08:11 PM, Daniel Vetter wrote:
> On Mon, May 09, 2016 at 08:16:07PM +0300, Ville Syrjälä wrote:
>> On Mon, May 09, 2016 at 05:08:43PM +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.
>>
>> This will also change the behaviour to block new readers while the
>> writer has the lock, whereas the old code would allow readers to
>> proceed in parallel. We do the whole hw counter + scanout position
>> query while holding the lock so it's not exactly zero amount of work,
>> but I'm not sure that's a real problem.
>>
>> I guess we could reduce the scope of the seqlock, but then maybe we'd
>> need to keep the vblank_time_lock spinlock as well. The details escape
>> me now, so I'd have re-read the code again.
>>
>> Ccing Mario too.
>
> Yeah, my idea was to keep the spinlock, and only replace the stuff in
> store_vblank and the few do {} while (cur_vblank != get_vblank_counter)
> loops. Extending the seqlock stuff to everything seems indeed counter to
> Mario's locking scheme.
>
> So goal would be to really just replace the half-baked seqlock that we
> have already, and leave all other locking unchanged.
> -Daniel
+1 to that, for simplicity. I thought Ville already had a patch laying
around somewhere which essentially does this?
-mario
>
>>
>>>
>>> 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>
>>> ---
>>> 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 */
>>> --
>>> 2.4.11
>>
>> --
>> Ville Syrjälä
>> Intel OTC
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
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