Re: [PATCH 1/3] drm: Defer disabling the vblank IRQ until the next interrupt (for instant-off)

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

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@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>

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@linux.intel.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 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

-- 
Ville Syrjälä
Intel OTC
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