On 03/15/2017 10:00 PM, Ville Syrjälä wrote:
On Wed, Mar 15, 2017 at 08:40:25PM +0000, Chris Wilson wrote:
On vblank instant-off systems, we can get into a situation where the cost
of enabling and disabling the vblank IRQ around a drmWaitVblank query
dominates. And with the advent of even deeper hardware sleep state,
touching registers becomes ever more expensive. However, we know that if
the user wants the current vblank counter, they are also very likely to
immediately queue a vblank wait and so we can keep the interrupt around
and only turn it off if we have no further vblank requests queued within
the interrupt interval.
After vblank event delivery, this patch adds a shadow of one vblank where
the interrupt is kept alive for the user to query and queue another vblank
event. Similarly, if the user is using blocking drmWaitVblanks, the
interrupt will be disabled on the IRQ following the wait completion.
However, if the user is simply querying the current vblank counter and
timestamp, the interrupt will be disabled after every IRQ and the user
will enabled it again on the first query following the IRQ.
v2: Mario Kleiner -
After testing this, one more thing that would make sense is to move
the disable block at the end of drm_handle_vblank() instead of at the
top.
Turns out that if high precision timestaming is disabled or doesn't
work for some reason (as can be simulated by echo 0 >
/sys/module/drm/parameters/timestamp_precision_usec), then with your
delayed disable code at its current place, the vblank counter won't
increment anymore at all for instant queries, ie. with your other
"instant query" patches. Clients which repeatedly query the counter
and wait for it to progress will simply hang, spinning in an endless
query loop. There's that comment in vblank_disable_and_save:
"* Skip this step if there isn't any high precision timestamp
* available. In that case we can't account for this and just
* hope for the best.
*/
With the disable happening after leading edge of vblank (== hw counter
increment already happened) but before the vblank counter/timestamp
handling in drm_handle_vblank, that step is needed to keep the counter
progressing, so skipping it is bad.
Now without high precision timestamping support, a kms driver must not
set dev->vblank_disable_immediate = true, as this would cause problems
for clients, so this shouldn't matter, but it would be good to still
make this robust against a future kms driver which might have
unreliable high precision timestamping, e.g., high precision
timestamping that intermittently doesn't work.
v3: Patch before coffee needs extra coffee.
Testcase: igt/kms_vblank
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Michel Dänzer <michel@xxxxxxxxxxx>
Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxxx>,
Cc: Mario Kleiner <mario.kleiner.de@xxxxxxxxx>
Yep. This seems like a good idea to me. I just neglected to review it
last time around (and maybe even before that?) for some reason. Locks
seem to be taken in the right order, so it at least looks safe to me.
Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Hi,
as a followup to this one, maybe we should move the
drm_handle_vblank_events(dev, pipe); down, immediately after Chris new
delayed disable code?
The idea was to avoid lots of redundant enable->disable->enable... calls
by having some 1 frame delay before disable. This works for pure vblank
count/ts queries.
But both DRI2 and DRI3/Present use vblank events to trigger a
pageflip-ioctl at the right target vblank. With the current ordering we
may dispatch the vblank swap trigger event to the X-Server and drop the
vblank refcount to zero due to the vblank_put inside
drm_handle_vblank_events for the dispatched event, then detect in this
patch that refcount == 0 and disable vblanks, but a few microseconds
later the server will queue a pageflip ioctl which bumps the refcount
and reenables vblank irqs, so we have a redundant disable->enable.
Also many kms drivers now use drm_crtc_arm_vblank_event() for pageflip
completion handling at vblank, the pageflip completion events are also
dispatched via drm_handle_vblank_events(). After a pageflip completes,
it makes sense to have this "swap shadow" of 1 full frame, as animations
would likely queue a new vblank query/event immediately for the next
animation frame.
-mario
---
drivers/gpu/drm/drm_irq.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 9bdca69f754c..e64b05ea95ea 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1198,9 +1198,9 @@ static void drm_vblank_put(struct drm_device *dev, unsigned int pipe)
if (atomic_dec_and_test(&vblank->refcount)) {
if (drm_vblank_offdelay == 0)
return;
- else if (dev->vblank_disable_immediate || drm_vblank_offdelay < 0)
+ else if (drm_vblank_offdelay < 0)
vblank_disable_fn((unsigned long)vblank);
- else
+ else if (!dev->vblank_disable_immediate)
mod_timer(&vblank->disable_timer,
jiffies + ((drm_vblank_offdelay * HZ)/1000));
}
@@ -1734,6 +1734,16 @@ bool drm_handle_vblank(struct drm_device *dev, unsigned int pipe)
wake_up(&vblank->queue);
drm_handle_vblank_events(dev, pipe);
+ /* With instant-off, we defer disabling the interrupt until after
+ * we finish processing the following vblank. The disable has to
+ * be last (after drm_handle_vblank_events) so that the timestamp
+ * is always accurate.
+ */
+ if (dev->vblank_disable_immediate &&
+ drm_vblank_offdelay > 0 &&
+ !atomic_read(&vblank->refcount))
+ vblank_disable_fn((unsigned long)vblank);
+
spin_unlock_irqrestore(&dev->event_lock, irqflags);
return true;
--
2.11.0
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