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

[Mario Kleiner]

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 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 | 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