[Intel-gfx] [PATCH 10/29] drm/i915/dp: Specify the FEC overhead as an increment vs. a remainder

[Imre Deak]

On Wed, Oct 25, 2023 at 06:27:58PM +0300, Ville Syrjälä wrote:
> On Tue, Oct 24, 2023 at 04:09:06AM +0300, Imre Deak wrote:
> > A follow-up patch will add up all the overheads on a DP link, where it
> > makes more sense to specify each overhead factor in terms of the added
> > overhead amount vs. the reciprocal remainder (of usable BW remaining
> > after deducting the overhead). Prepare for that here, keeping the
> > existing behavior.
> > 
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> > index 2048649b420b2..0c0f026fb3161 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > @@ -85,8 +85,8 @@
> >  #define DP_DSC_MAX_ENC_THROUGHPUT_0		340000
> >  #define DP_DSC_MAX_ENC_THROUGHPUT_1		400000
> >  
> > -/* DP DSC FEC Overhead factor = 1/(0.972261) */
> > -#define DP_DSC_FEC_OVERHEAD_FACTOR		972261
> 
> Does anyone know where this magic number comes from?
> 
> AFAICS we should have 250 LL + 5 FEC_PARITY_PH + 1 CD_ADJ, which
> gives us the 256/250 = 2.4% number. In addition there's the
> extra parity marker symbol insterted every 128 FEC blocks,
> which makes the total overhead 2.4015625%, which is still
> not that magic number.

IIRC it's mentioned in the bpsec audio programming sequence, but the
rational for it isn't detailed. I suppose it could be the 2.4% FEC
overhead + a fixed overhead for DSC EOC symbols; but the latter is not
really fixed, it depends on the video mode/slice count etc. as I
commented on later.

> 
> > +/* DP DSC FEC Overhead factor = 1/(0.972261) = 1.028530 ppm */
> > +#define DP_DSC_FEC_OVERHEAD_FACTOR		1028530
> >  
> >  /* Compliance test status bits  */
> >  #define INTEL_DP_RESOLUTION_SHIFT_MASK	0
> > @@ -680,8 +680,8 @@ int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
> >  
> >  u32 intel_dp_mode_to_fec_clock(u32 mode_clock)
> >  {
> > -	return div_u64(mul_u32_u32(mode_clock, 1000000U),
> > -		       DP_DSC_FEC_OVERHEAD_FACTOR);
> > +	return div_u64(mul_u32_u32(mode_clock, DP_DSC_FEC_OVERHEAD_FACTOR),
> > +		       1000000U);
> >  }
> >  
> >  static int
> > -- 
> > 2.39.2
> 
> -- 
> Ville Syrjälä
> Intel