On Mon, Apr 04, 2022 at 04:49:18PM +0300, Vinod Govindapillai wrote:
> In configurations with single DRAM channel, for usecases like
> 4K 60 Hz, FIFO underruns are observed quite frequently. Looks
> like the wm0 watermark values need to bumped up because the wm0
> memory latency calculations are probably not taking the DRAM
> channel's impact into account.
> 
> As per the Bspec 49325, if the ddb allocation can hold at least
> one plane_blocks_per_line we should have selected method2.
> Assuming that modern HW versions have enough dbuf to hold
> at least one line, set the wm blocks to equivalent to blocks
> per line.
> 
> cc: Ville Syrjälä <ville.syrj...@linux.intel.com>
> cc: Stanislav Lisovskiy <stanislav.lisovs...@intel.com>
> 
> Signed-off-by: Vinod Govindapillai <vinod.govindapil...@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_pm.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 8824f269e5f5..ae28a8c63ca4 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -5474,7 +5474,24 @@ static void skl_compute_plane_wm(const struct 
> intel_crtc_state *crtc_state,
>               }
>       }
>  
> -     blocks = fixed16_to_u32_round_up(selected_result) + 1;
> +     /*
> +      * Lets have blocks at minimum equivalent to plane_blocks_per_line
> +      * as there will be at minimum one line for lines configuration.
> +      *
> +      * As per the Bspec 49325, if the ddb allocation can hold at least
> +      * one plane_blocks_per_line, we should have selected method2 in
> +      * the above logic. Assuming that modern versions have enough dbuf
> +      * and method2 guarantees blocks equivalent to at least 1 line,
> +      * select the blocks as plane_blocks_per_line.
> +      *
> +      * TODO: Revisit the logic when we have better understanding on DRAM
> +      * channels' impact on the level 0 memory latency and the relevant
> +      * wm calculations.
> +      */
> +     blocks = skl_wm_has_lines(dev_priv, level) ?
> +                     max_t(u32, fixed16_to_u32_round_up(selected_result) + 1,
> +                               
> fixed16_to_u32_round_up(wp->plane_blocks_per_line)) :
> +                     fixed16_to_u32_round_up(selected_result) + 1;
>       lines = div_round_up_fixed16(selected_result,
>                                    wp->plane_blocks_per_line);

I think this is a good fix, no IGT/BAT regressions are visible, also 
it fixes some of the current issues at customer side. So don't see any reason
for it not to be merged.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovs...@intel.com>

P.S: there is some checkpatch warning, which probably needs to be addressed :)

Stan

>  
> -- 
> 2.25.1
> 

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