[Intel-gfx] [PATCH] drm/i915/tgl/psr: Fix glitches when doing frontbuffer modifications

[Mun, Gwan-gyeong]

1. While testing the problematic scenario, it has not always shown the
IOMMU DAMR related below errors on the drm-tip. 
   (sometimes the error messages raised, but some times it has not
happened on the same kernel and scenario.
  
DMAR: DRHD: handling fault status reg 2
DMAR: [DMA Read] Request device [00:02.0] PASID 0xffffffff fault addr
0xfc001000 [fault reason 06] PTE Read access is not set
DMAR: DRHD: handling fault status reg 3
DMAR: [DMA Read] Request device [00:02.0] PASID 0xffffffff fault addr
0xfc000000 [fault reason 06] PTE Read access is not set

2  After applying this patch the screen glitch issues have been
remarkably alleviated.
  - Eventhough there infrequently showed the screen glitch issues.
  - But I agree to apply this patch as a workaround by adding the
explanation below.
 
3. The dc state and PSR enable/disable scenarios has been changed by
this patch.
      
(1)Before applying patch
  enable psr 
    -> (front buffer updates) 
        -> intel_psr_flush 
                  ^   -> psr_force_hw_tracking_exit()
                  |                   : write CURSURFLIVE 
                  |                          |
                  |  (front buffer updates)  |
                  +--------------------------+

    PSR enabled -------------------------------------- -->
   ( DC state controlled by DMC firmware)
                                   

 (2) After applying patch
  enable psr 
   ^  -> (front buffer updates) 
   |       -> intel_psr_flush
   |          -> psr_force_hw_tracking_exit()
   |               : call intel_psr_exit()
   |                                 -> disable psr
   |                                         |
   |                                         |
   +-----------------------------------------+

PSR enabled ---------------------------> PSR disabled
  ^                                          |
  |                                          |
  +------------------------------------------+
   ( DC state controlled by DMC firmware)

the repeating of enabling and disabling of PSR by the rapid screen
updates prevents entering of low power dc states.
Infereing from this scenario, it indirectly touches DC state and it
alleviates the issue.


On Fri, 2020-10-02 at 16:16 -0700, José Roberto de Souza wrote:
> Writes to CURSURFLIVE in TGL are causing IOMMU errors and visual
> glitches that are often reproduced when executing CPU intensive
> workloads while a eDP 4K panel is attached.
> 
> Manually exiting PSR causes the frontbuffer to be updated without
> glitches and the IOMMU errors are also gone but this comes at the
> cost
> of less time with PSR active.
> 
> So using this workaround until this issue is root caused and a better
> fix is found.
> 
> The current code is already ready to enable PSR after this exit if
> there is not other frontbuffer modifications.
> 
> Adding a new if block in psr_force_hw_tracking_exit() instead of
> reuse
> the else/gen8- block because the plan is to revert this workaround
> as soon as a better solution is found.
> 
> Cc: Gwan-gyeong Mun <gwan-gyeong.mun at intel.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Signed-off-by: José Roberto de Souza <jose.souza at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_psr.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_psr.c
> b/drivers/gpu/drm/i915/display/intel_psr.c
> index 8a9d0bdde1bf..8630121dbbbe 100644
> --- a/drivers/gpu/drm/i915/display/intel_psr.c
> +++ b/drivers/gpu/drm/i915/display/intel_psr.c
> @@ -1152,7 +1152,21 @@ void intel_psr_disable(struct intel_dp
> *intel_dp,
>  
>  static void psr_force_hw_tracking_exit(struct drm_i915_private
> *dev_priv)
>  {
> -	if (INTEL_GEN(dev_priv) >= 9)
> +	if (IS_TIGERLAKE(dev_priv))
> +		/*
> +		 * Writes to CURSURFLIVE in TGL are causing IOMMU
> errors and
> +		 * visual glitches that are often reproduced when
> executing
> +		 * CPU intensive workloads while a eDP 4K panel is
> attached.
> +		 *
> +		 * Manually exiting PSR causes the frontbuffer to be
> updated
> +		 * without glitches and the IOMMU errors are also gone
> but
> +		 * this comes at the cost of less time with PSR active.
> +		 *
> +		 * So using this workaround until this issue is root
> caused
> +		 * and a better fix is found.
> +		 */
> +		intel_psr_exit(dev_priv);
> +	else if (INTEL_GEN(dev_priv) >= 9)
>  		/*
>  		 * Display WA #0884: skl+
>  		 * This documented WA for bxt can be safely applied