Re: [PATCH 2/3] drm/i915/dp: Fix sink-crc reads.

[Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>]

On Tue, 2018-04-24 at 16:26 +0300, Mika Kahola wrote:
> On Mon, 2018-04-23 at 19:56 -0700, Dhinakaran Pandiyan wrote:
> > Sink crc is calculated by the sink for static frames irrespective of
> > what the driver sets in TEST_SINK_START dpcd. Since PSR is the only
> > use
> > case for sink crc, we don't really need the sink_crc_{start, stop}
> > code.
> > 
> > The second problem with the current implementation is vblank waits.
> > Enabling vblank interrupts triggers PSR exit, which means we aren't
> > really reading the correct CRC values for PSR tests. vblank waits are
> > replaced by delays.
> > 
> > With the changes made in this patch, sink CRC is available only for
> > static frames. I have tested this on a SKL laptop with PSR panel.
> > 
> > Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_debugfs.c |   2 +-
> >  drivers/gpu/drm/i915/intel_dp.c     | 114 ++++--------------------
> > ------------
> >  drivers/gpu/drm/i915/intel_drv.h    |   3 +-
> >  3 files changed, 15 insertions(+), 104 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_debugfs.c
> > b/drivers/gpu/drm/i915/i915_debugfs.c
> > index 2f05f5262bba..35fa1418cc07 100644
> > --- a/drivers/gpu/drm/i915/i915_debugfs.c
> > +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> > @@ -2784,7 +2784,7 @@ static int i915_sink_crc(struct seq_file *m,
> > void *data)
> >  
> >  		intel_dp = enc_to_intel_dp(state->best_encoder);
> >  
> > -		ret = intel_dp_sink_crc(intel_dp, crtc_state, crc);
> > +		ret = intel_dp_sink_crc(intel_dp, crc);
> >  		if (ret)
> >  			goto err;
> >  
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c
> > b/drivers/gpu/drm/i915/intel_dp.c
> > index 7dcc874b7d8f..7352ab631ea8 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -3876,32 +3876,18 @@ intel_dp_configure_mst(struct intel_dp
> > *intel_dp)
> >  					intel_dp->is_mst);
> >  }
> >  
> > -static int intel_dp_sink_crc_stop(struct intel_dp *intel_dp,
> > -				  struct intel_crtc_state
> > *crtc_state, bool disable_wa)
> > +int intel_dp_sink_crc(struct intel_dp *intel_dp, u8 *crc)
> >  {
> > -	struct intel_digital_port *dig_port =
> > dp_to_dig_port(intel_dp);
> > -	struct drm_i915_private *dev_priv = to_i915(dig_port-
> > >base.base.dev);
> > -	struct intel_crtc *intel_crtc = to_intel_crtc(crtc_state-
> > >base.crtc);
> > -	u8 buf;
> > -	int ret = 0;
> > -	int count = 0;
> > -	int attempts = 10;
> > +	int count = 0, ret = 0, attempts;
> >  
> > -	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_TEST_SINK, &buf) <
> > 0) {
> > -		DRM_DEBUG_KMS("Sink CRC couldn't be stopped
> > properly\n");
> > -		ret = -EIO;
> > -		goto out;
> > -	}
> > +	for (attempts = 0; attempts < 3 && count == 0; attempts++) {
> A question. How did you end up with three attempts or one and a half
> frames? We used to try 6 times or 6 vblanks until we gave up trying.
> Well, I don't really know the history why we have ended up with 6
> attempts in the first place. 

CRC should be available within the next static frame. So the idea for a
tests is to do, for example, a page flip and immediately attempt to read
the CRC. Since we cannot exactly synchronize the flip getting committed,
debugfs read and CRC generation, all that the kernel can do correctly is
check if the CRC is available within a frame and read it.

I think the reason for trying 6 times was to somehow get a non-zero CRC.
But that means tests end up reading CRC of a different frame.


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