Re: [PATCH 1/2] drm/i915: Do not enable FEC without DSC

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Mar 26, 2019 at 09:00:27AM -0700, Manasi Navare wrote:
> On Tue, Mar 26, 2019 at 04:49:02PM +0200, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > Currently we enable FEC even when DSC is no used. While that is
> > theoretically valid supposedly there isn't much of a benefit from
> > this. But more importantly we do not account for the FEC link
> > bandwidth overhead (2.4%) in the non-DSC link bandwidth computations.
> > So the code may think we have enough bandwidth when we in fact
> > do not.
> > 
> > Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
> > Cc: Manasi Navare <manasi.d.navare@intel.comk>
> > Fixes: 240999cf339f ("i915/dp/fec: Add fec_enable to the crtc state.")
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index 326de12c3f44..bbf678561509 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -1925,6 +1925,9 @@ static int intel_dp_dsc_compute_config(struct intel_dp *intel_dp,
> >  	int pipe_bpp;
> >  	int ret;
> >  
> > +	pipe_config->fec_enable = !intel_dp_is_edp(intel_dp) &&
> > +		intel_dp_supports_fec(intel_dp, pipe_config);
> > +
> 
> We could still not enable DSC after this point since it has more checks in this
> function. Even though in that case we would fail the encoder config so wouldnt
> matter if we have enabled FEC or not, but its less intutive.
> IMHO, the ideal place to set the fec enable is in intel_dp_compute_link_config()
> after the all to dsc_compute_config and set it only if pipe_config->dsc_params.compression_enable

That would require changing intel_dp_supports_dsc() which I decided
wasn't worth the hassle.

> 
> Manasi
> 
> >  	if (!intel_dp_supports_dsc(intel_dp, pipe_config))
> >  		return -EINVAL;
> >  
> > @@ -2168,9 +2171,6 @@ intel_dp_compute_config(struct intel_encoder *encoder,
> >  	if (adjusted_mode->flags & DRM_MODE_FLAG_DBLCLK)
> >  		return -EINVAL;
> >  
> > -	pipe_config->fec_enable = !intel_dp_is_edp(intel_dp) &&
> > -				  intel_dp_supports_fec(intel_dp, pipe_config);
> > -
> >  	ret = intel_dp_compute_link_config(encoder, pipe_config, conn_state);
> >  	if (ret < 0)
> >  		return ret;
> > -- 
> > 2.19.2
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Ville Syrjälä
Intel
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