Re: [PATCH AUTOSEL for 4.4 016/115] drm: Defer disabling the vblank IRQ until the next interrupt (for instant-off)

[Ben Hutchings <ben.hutchings@codethink.co.uk>]

On Sat, 2018-03-03 at 22:30 +0000, Sasha Levin wrote:
> From: Chris Wilson <chris@chris-wilson.co.uk>
> 
> [ Upstream commit 608b20506941969ea30d8c08dc9ae02bb87dbf7d ]

I think we need this one too:

commit 75cff0837c14eaf632efabb8d7ab9eec6394d20d
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Mar 24 17:30:58 2017 +0000

    drm: Make the decision to keep vblank irq enabled earlier

Ben.

> 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@chris-wilson.co.uk
> Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
> ---
>  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 8090989185b2..4ddbc49125cd 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -1271,9 +1271,9 @@ 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));
>  	}
> @@ -1902,6 +1902,16 @@ 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;
> -- 
> 2.14.1
-- 
Ben Hutchings
Software Developer, Codethink Ltd.