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

[Ville Syrjälä]

On Thu, Mar 29, 2018 at 05:32:13PM +0100, Ben Hutchings wrote:
> On Sat, 2018-03-03 at 22:30 +0000, Sasha Levin wrote:
> > From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > 
> > [ Upstream commit 608b20506941969ea30d8c08dc9ae02bb87dbf7d ]
> 
> I think we need this one too:
> 
> commit 75cff0837c14eaf632efabb8d7ab9eec6394d20d
> Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Date:   Fri Mar 24 17:30:58 2017 +0000
> 
>     drm: Make the decision to keep vblank irq enabled earlier

Why are we backporting optimizations anyway?

> 
> 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@xxxxxxxxxxxxxxxxxx>
> > Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > Cc: Daniel Vetter <daniel@xxxxxxxx>
> > Cc: Michel Dänzer <michel@xxxxxxxxxxx>
> > Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> > Cc: Dave Airlie <airlied@xxxxxxxxxx>,
> > Cc: Mario Kleiner <mario.kleiner.de@xxxxxxxxx>
> > Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Link: http://patchwork.freedesktop.org/patch/msgid/20170315204027.20160-1-chris@xxxxxxxxxxxxxxxxxx
> > Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
> > ---
> >  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.

-- 
Ville Syrjälä
Intel OTC