[PATCH] drm: Make the decision to keep vblank irq enabled earlier

[Ville Syrjälä]

On Thu, Mar 23, 2017 at 07:51:06AM +0000, Chris Wilson wrote:
> We want to provide the vblank irq shadow for pageflip events as well as
> vblank queries. Such events are completed within the vblank interrupt
> handler, and so the current check for disabling the irq will disable it
> from with the same interrupt as the last pageflip event. If we move the
> decision on whether to disable the irq (based on there no being no
> remaining vblank events, i.e. vblank->refcount == 0) to before we signal
> the events, we will only disable the irq on the interrupt after the last
> event was signaled. In the normal course of events, this will keep the
> vblank irq enabled for the entire flip sequence whereas before it would
> flip-flop around every interrupt.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Michel Dänzer <michel at daenzer.net>
> Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Cc: Dave Airlie <airlied at redhat.com>,
> Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
> ---
>  drivers/gpu/drm/drm_irq.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 5b77057e91ca..1d6bcee3708f 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -1741,6 +1741,7 @@ bool drm_handle_vblank(struct drm_device *dev, unsigned int pipe)
>  {
>  	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
>  	unsigned long irqflags;
> +	bool disable_irq;
>  
>  	if (WARN_ON_ONCE(!dev->num_crtcs))
>  		return false;
> @@ -1768,16 +1769,19 @@ bool drm_handle_vblank(struct drm_device *dev, unsigned int pipe)
>  	spin_unlock(&dev->vblank_time_lock);
>  
>  	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.
> +	 * we finish processing the following vblank after all events have
> +	 * been signaled. The disable has to be last (after
> +	 * drm_handle_vblank_events) so that the timestamp is always accurate.

We wouldn't actually do the disable as long there's a reference still
held, so the timestamp should be fine in that case. And if there aren't
any references the timestamp shouldn't matter... I think. But it's
probably more clear to keep to the order you propose here anyway.

Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Oh, and now that I think about this stuff again, I start to wonder why
I made the disable actually update the seq/ts. If the interrupt is
currently enabled the seq/ts should be reasonably uptodate already
when we do disable the interrupt. Perhaps I was only thinking about
drm_vblank_off() when I made that change, or I decided that I didn't
want two different disable codepaths. Anyways, just an idea that
we might be able to make the vblank irq disable a little cheaper.

>  	 */
> -	if (dev->vblank_disable_immediate &&
> -	    drm_vblank_offdelay > 0 &&
> -	    !atomic_read(&vblank->refcount))
> +	disable_irq = (dev->vblank_disable_immediate &&
> +		       drm_vblank_offdelay > 0 &&
> +		       !atomic_read(&vblank->refcount));
> +
> +	drm_handle_vblank_events(dev, pipe);
> +
> +	if (disable_irq)
>  		vblank_disable_fn((unsigned long)vblank);
>  
>  	spin_unlock_irqrestore(&dev->event_lock, irqflags);
> -- 
> 2.11.0

-- 
Ville Syrjälä
Intel OTC