This is a note to let you know that I've just added the patch titled
drm: Defer disabling the vblank IRQ until the next interrupt (for instant-off)
to the 3.18-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: drm-defer-disabling-the-vblank-irq-until-the-next-interrupt-for-instant-off.patch and it can be found in the queue-3.18 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From foo@baz Mon Mar 19 10:11:52 CET 2018
From: Chris Wilson chris@chris-wilson.co.uk Date: Wed, 15 Mar 2017 20:40:25 +0000 Subject: drm: Defer disabling the vblank IRQ until the next interrupt (for instant-off)
From: Chris Wilson chris@chris-wilson.co.uk
[ Upstream commit 608b20506941969ea30d8c08dc9ae02bb87dbf7d ]
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@chris-wilson.co.uk Cc: Ville Syrjälä ville.syrjala@linux.intel.com Cc: Daniel Vetter daniel@ffwll.ch Cc: Michel Dänzer michel@daenzer.net Cc: Laurent Pinchart laurent.pinchart@ideasonboard.com Cc: Dave Airlie airlied@redhat.com, Cc: Mario Kleiner mario.kleiner.de@gmail.com Reviewed-by: Ville Syrjälä ville.syrjala@linux.intel.com Signed-off-by: Daniel Vetter daniel.vetter@ffwll.ch Link: http://patchwork.freedesktop.org/patch/msgid/20170315204027.20160-1-chris@ch... Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/gpu/drm/drm_irq.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-)
--- a/drivers/gpu/drm/drm_irq.c +++ b/drivers/gpu/drm/drm_irq.c @@ -1038,9 +1038,9 @@ void drm_vblank_put(struct drm_device *d 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)); } @@ -1664,6 +1664,16 @@ bool drm_handle_vblank(struct drm_device wake_up(&vblank->queue); drm_handle_vblank_events(dev, crtc);
+ /* 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;
Patches currently in stable-queue which might be from chris@chris-wilson.co.uk are
queue-3.18/drm-defer-disabling-the-vblank-irq-until-the-next-interrupt-for-instant-off.patch