On 04/15/2015 03:03 AM, Mario Kleiner wrote:
On 04/02/2015 01:34 PM, 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. 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 in the interrupt interval.
After vblank event delivery there is 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.
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>
---
drivers/gpu/drm/drm_irq.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index c8a34476570a..6f5dc18779e2 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1091,9 +1091,9 @@ void drm_vblank_put(struct drm_device *dev, int
crtc)
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));
}
@@ -1697,6 +1697,17 @@ bool drm_handle_vblank(struct drm_device *dev,
int crtc)
spin_lock_irqsave(&dev->event_lock, irqflags);
You could move the code before the spin_lock_irqsave(&dev->event_lock,
irqflags); i think it doesn't need that lock?
+ if (dev->vblank_disable_immediate &&
!atomic_read(&vblank->refcount)) {
Also check for (drm_vblank_offdelay > 0) to make sure we have a way out
of instant disable here, and the same meaning of of drm_vblank_offdelay
like we have in the current implementation.
This hunk ...
+ unsigned long vbl_lock_irqflags;
+
+ spin_lock_irqsave(&dev->vbl_lock, vbl_lock_irqflags);
+ if (atomic_read(&vblank->refcount) == 0 && vblank->enabled) {
+ DRM_DEBUG("disabling vblank on crtc %d\n", crtc);
+ vblank_disable_and_save(dev, crtc);
+ }
+ spin_unlock_irqrestore(&dev->vbl_lock, vbl_lock_irqflags);
... is the same as a call to vblank_disable_fn((unsigned long) vblank);
Maybe replace by that call?
You could also return here already, as the code below will just take a
lock, realize vblanks are now disabled and then release the locks and exit.
+ }
+
/* Need timestamp lock to prevent concurrent execution with
* vblank enable/disable, as this would cause inconsistent
* or corrupted timestamps and vblank counts.
I think the logic itself is fine and at least basic testing of the patch
on a Intel HD Ironlake didn't show problems, so with the above taken
into account it would have my slightly uneasy reviewed-by.
One thing that worries me a little bit about the disable inside vblank
irq are the potential races between the disable code and the display
engine which could cause really bad off-by-one errors for clients on a
imperfect driver. These races can only happen if vblank enable or
disable happens close to or inside the vblank. This approach lets the
instant disable happen exactly inside vblank when there is the highest
chance of triggering that condition.
This doesn't seem to be a problem for intel kms, but other drivers don't
have instant disable yet, so we don't know how well we could do it
there. Additionally things like dynamic power management tend to operate
inside vblank, sometimes with "funny" side effects to other stuff, e.g.,
dpm on AMD, as i remember from some long debug session with Michel and
Alex last summer where dpm played a role. Therefore it seems more safe
to me to avoid actions inside vblank that could be done outside. E.g.,
instead of doing the disable inside the vblank irq one could maybe just
schedule an exact timer to do the disable a few milliseconds later in
the middle of active scanout to avoid these potential issues?
-mario
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.
For reference, my tweak of your patch looks now like this for the hunk
at the bottom of drm_handle_vblank():
....
spin_unlock(&dev->vblank_time_lock);
wake_up(&vblank->queue);
drm_handle_vblank_events(dev, crtc);
+ if (dev->vblank_disable_immediate && drm_vblank_offdelay > 0 &&
+ !atomic_read(&vblank->refcount)) {
+ unsigned long vbl_lock_irqflags;
+
+ spin_lock_irqsave(&dev->vbl_lock, vbl_lock_irqflags);
+ if (atomic_read(&vblank->refcount) == 0 &&
vblank->enabled) {
+ DRM_DEBUG("disabling vblank on crtc %d\n", crtc);
+ vblank_disable_and_save(dev, crtc);
+ }
+ spin_unlock_irqrestore(&dev->vbl_lock, vbl_lock_irqflags);
+ }
+
spin_unlock_irqrestore(&dev->event_lock, irqflags);
return true;
This could be further simplified to
......
spin_unlock(&dev->vblank_time_lock);
wake_up(&vblank->queue);
drm_handle_vblank_events(dev, crtc);
+
+ if (dev->vblank_disable_immediate && drm_vblank_offdelay > 0 &&
+ !atomic_read(&vblank->refcount)) {
+ vblank_disable_fn(vblank);
+ }
+
spin_unlock_irqrestore(&dev->event_lock, irqflags);
return true;
The tweaked version worked fine in all my tests.
-mario
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