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

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

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@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.

> +/* 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