[Intel-gfx] [PATCH v2] drm/i915: Wait for PSR exit before checking for vblank evasion

[Tarun Vyas]

On Tue, Jun 19, 2018 at 02:59:54PM -0700, Tarun Vyas wrote:
> On Tue, Jun 19, 2018 at 02:54:07PM -0700, Dhinakaran Pandiyan wrote:
> > On Tue, 2018-06-19 at 14:27 -0700, Dhinakaran Pandiyan wrote:
> > > On Mon, 2018-05-14 at 13:49 -0700, Tarun Vyas wrote:
> > > > 
> > > > The PIPEDSL freezes on PSR entry and if PSR hasn't fully exited,
> > > > then
> > > > the pipe_update_start call schedules itself out to check back
> > > > later.
> > > > 
> > > > On ChromeOS-4.4 kernel, which is fairly up-to-date w.r.t drm/i915
> > > > but
> > > > lags w.r.t core kernel code, hot plugging an external display
> > > > triggers
> > > > tons of "potential atomic update errors" in the dmesg, on *pipe A*.
> > > > A
> > > > closer analysis reveals that we try to read the scanline 3 times
> > > > and
> > > > eventually timeout, b/c PSR hasn't exited fully leading to a
> > > > PIPEDSL
> > > > stuck @ 1599. This issue is not seen on upstream kernels, b/c for
> > > > *some*
> > > > reason we loop inside intel_pipe_update start for ~2+ msec which in
> > > > this
> > > > case is more than enough to exit PSR fully, hence an *unstuck*
> > > > PIPEDSL
> > > > counter, hence no error. On the other hand, the ChromeOS kernel
> > > > spends
> > > > ~1.1 msec looping inside intel_pipe_update_start and hence errors
> > > > out
> > > > b/c the source is still in PSR.
> > > > 
> > > > Regardless, we should wait for PSR exit (if PSR is supported and
> > > > active
> > > > on the current pipe) before reading the PIPEDSL, b/c if we haven't
> > > > fully exited PSR, then checking for vblank evasion isn't actually
> > > > applicable.
> > > > 
> > > > This scenario applies to a configuration with an additional pipe,
> > > > as of now
> > > > 
> > > > Signed-off-by: Tarun Vyas <tarun.vyas at intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/intel_sprite.c | 7 +++++--
> > > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/intel_sprite.c
> > > > b/drivers/gpu/drm/i915/intel_sprite.c
> > > > index ee23613f9fd4..481d310e5c3b 100644
> > > > --- a/drivers/gpu/drm/i915/intel_sprite.c
> > > > +++ b/drivers/gpu/drm/i915/intel_sprite.c
> > > > @@ -107,14 +107,17 @@ void intel_pipe_update_start(const struct
> > > > intel_crtc_state *new_crtc_state)
> > > >  						      VBLANK_EVASI
> > > > ON
> > > > _TIME_US);
> > > >  	max = vblank_start - 1;
> > > >  
> > > > -	local_irq_disable();
> > > > -
> > > >  	if (min <= 0 || max <= 0)
> > > >  		return;
> > > >  
> > > >  	if (WARN_ON(drm_crtc_vblank_get(&crtc->base)))
> > > >  		return;
> > > >  
> > > > +	if(new_crtc_state->has_psr && dev_priv->psr.active)
> > > > +		psr_wait_for_idle(dev_priv);
> > > How about just waiting for PSR_STATUS to idle without grabbing any
> > > locks or checking whether PSR is active?
> > > 
> > > Status should be idle if PSR was disabled or on it's way to becoming
> > > idle if it was enabled. Even if PSR did get enabled while we are in
> > > pipe_update_start(), it will not be active as long as VBIs are
> > > enabled.
> > > 
> Right, if we are OK with some duplication (of psr_wait_for_idle) inside intel_psr.c, then we can duplicate the PSR2 vs. PSR check that's being done in psr_wait_for_idle and then just wait without grabbing any locks, so essentially a lockless version of psr_wait_for_idle()
> > Correct me if this was already considered, why not wait until the
> > scanline counter starts moving? I see we have a 
> > 	intel_wait_for_pipe_scanline_moving(crtc) that's used when the
> > pipe is enabled.
> > 
> > -DK
> 
> Didn't consider this before, but, pipe_scanline_is_moving waits for a minimum of 5 msec. Are we OK with a min wait of 5 msec inside pipe_update_start ? Heuristically, waiting for PSR idle has almost always returned within < 2 msec. Occasionally it takes upto 1 full frame.
As per some preliminary measurements
Approach 1:
	Wait *unconditionally* (so no need to check for PSR enabled/disabled and hence no locks) for PSR_STATUS to IDLE out. 
This takes ~7msec when PSR is active and ~2 usec when PSR is inactive/disabled.

Approach 2:
	Use intel_wait_for_pipe_scanline_moving to wait for PIPEDSL to start moving after PSR exit. Currently, this ends up waiting for a minimum of 5 msec but I changed this to accept a caller defined value for the delay.
After the above changes, this approach takes ~7msec when PSR is active and ~25-40 usec with PSR disabled, b/c we still need to check for at least 10+ usec and see if PIPEDSL moved, if it did, we wait for longer, otherwise we move on.