Re: [PATCH] drm/i915/psr: Display WA 0884 applied broadly for more HW tracking.

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Mon, Mar 12, 2018 at 11:40:26AM -0700, Pandiyan, Dhinakaran wrote:
> 
> 
> 
> On Mon, 2018-03-12 at 20:29 +0200, Ville Syrjälä wrote:
> > On Thu, Mar 08, 2018 at 04:52:18PM -0800, Rodrigo Vivi wrote:
> > > WA 0884:bxt:all,cnl:*:A - "When FBC is enabled with eDP PSR,
> > > the CPU host modify writes may not get updated on the Display
> > > as expected.
> > > WA: Write 0x00000000 to CUR_SURFLIVE_A with every CPU
> > > host modify write to trigger PSR exit."
> > > 
> > > We can also find on spec other cases where they describe
> > > bogus writes to cursor registers to force PSR exit with
> > > HW tracking. And it was confirmed by HW engineers that
> > > this Wa can be safely applied for any frontbuffer activity.
> > > 
> > > So let's use this more and more here instead of forcibly
> > > disable and re-enable PSR everytime that we have a simple
> > > reliable flush case.
> > > 
> > > Other commits improve the fbcon/fbdev use a lot, but this
> > > approach is the only when where we can get a fully reliable
> > > console with no slowness or missed frames and PSR still
> > > enabled and active.
> > > 
> > > v2: - Rebase on drm-tip
> > >     - (DK) Add a comment to explain that WA
> > >     tells about writing 0 to CUR_SURFLIVE_A but we write to
> > >     CUR_SURFLIVE(pipe).
> > > v3: Wa doesn't work on PSR2.
> > > 
> > > Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
> > > Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/i915_reg.h  |  3 +++
> > >  drivers/gpu/drm/i915/intel_psr.c | 19 +++++++++++++++++--
> > >  2 files changed, 20 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> > > index e6a8c0ee7df1..abdc513a9edd 100644
> > > --- a/drivers/gpu/drm/i915/i915_reg.h
> > > +++ b/drivers/gpu/drm/i915/i915_reg.h
> > > @@ -6032,6 +6032,9 @@ enum {
> > >  #define IVB_CURSOR_B_OFFSET 0x71080
> > >  #define IVB_CURSOR_C_OFFSET 0x72080
> > >  
> > > +#define _CUR_SURLIVE		0x700AC
> > > +#define CUR_SURLIVE(pipe)	_CURSOR2(pipe, _CUR_SURLIVE)
> > 
> > There should be a better place for this.

Oh! sorry... my inbox refreshed after I pushed the button :(
I tried to keep near the cursor regs. Do you want me to move in a
follow-up patch? where it would be better?

> > 
> > > +
> > >  /* Display A control */
> > >  #define _DSPACNTR				0x70180
> > >  #define   DISPLAY_PLANE_ENABLE			(1<<31)
> > > diff --git a/drivers/gpu/drm/i915/intel_psr.c b/drivers/gpu/drm/i915/intel_psr.c
> > > index 30932527e663..b0286722a72f 100644
> > > --- a/drivers/gpu/drm/i915/intel_psr.c
> > > +++ b/drivers/gpu/drm/i915/intel_psr.c
> > > @@ -1027,8 +1027,23 @@ void intel_psr_flush(struct drm_i915_private *dev_priv,
> > >  	dev_priv->psr.busy_frontbuffer_bits &= ~frontbuffer_bits;
> > >  
> > >  	/* By definition flush = invalidate + flush */
> > > -	if (frontbuffer_bits)
> > > -		intel_psr_exit(dev_priv);
> > > +	if (frontbuffer_bits) {
> > > +		if (dev_priv->psr.psr2_support ||
> > > +		    IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv)) {
> > > +			intel_psr_exit(dev_priv);
> > > +		} else {
> > > +			/*
> > > +			 * Display WA #0884: all
> > > +			 * This documented WA for bxt can be safely applied
> > > +			 * broadly so we can force HW tracking to exit PSR
> > > +			 * instead of disabling and re-enabling.
> > > +			 * Workaround tells us to write 0 to CUR_SURLIVE_A,
> > > +			 * but it makes more sense write to the current active
> > > +			 * pipe.
> > > +			 */
> > 
> > Might want to note that SURFLIVE is read only so the write should not
> > not have any other side effects, or at least I hope that is the case.
> > 
> > I don't really understand why we're doing PSR exit in the frontbuffer
> > flush though. Shouldn't we have already exited on invalidate?
> 
> fbdev code doesn't call invalidate at all and cannot call because the
> buffer modification functions are all in atomic contexts.

That was a design decision Paulo, and/or Chris, and/or Daniel made on
frontbuffer tracking engine, a while ago. By definition:
flush = invalidate + flush.
Otherwise in all fbdev code we would have to have a call together
of invalidate and flush.
We probably lack on documentation there...

> 
> 
> > 
> > > +			I915_WRITE(CUR_SURLIVE(pipe), 0);
> > > +		}
> > > +	}
> > >  
> > >  	if (!dev_priv->psr.active && !dev_priv->psr.busy_frontbuffer_bits)
> > >  		if (!work_busy(&dev_priv->psr.work.work))
> > > -- 
> > > 2.13.6
> > > 
> > > _______________________________________________
> > > Intel-gfx mailing list
> > > Intel-gfx@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> > 
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