[PATCH v2] drm/i915/display: Use static values for min_hblank

[Imre Deak]

On Tue, Jun 24, 2025 at 03:29:27PM +0530, Arun R Murthy wrote:
> Use recommended static values as per wa_14021694213 for min_hblank to
> avoid underruns.
> 
> v2: corrected checkpatch warning and retain the restriction for
> min_hblank (Jani)
> 
> Bspec: 74379
> Signed-off-by: Arun R Murthy <arun.r.murthy at intel.com>
> ---
> Changes in v2:
> - EDITME: describe what is new in this series revision.
> - EDITME: use bulletpoints and terse descriptions.
> - Link to v1: https://lore.kernel.org/r/20250624-min_hblank-v1-1-5c100e4a8b05@intel.com
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 62 ++++++---------------------------
>  1 file changed, 10 insertions(+), 52 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index f48912f308df7fd26c9d089e8f1bd096bfc8df95..b657340648cd2be72cec3fae606c0bc2ec8dba3d 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -3115,22 +3115,7 @@ int intel_dp_compute_min_hblank(struct intel_crtc_state *crtc_state,
>  				const struct drm_connector_state *conn_state)
>  {
>  	struct intel_display *display = to_intel_display(crtc_state);
> -	const struct drm_display_mode *adjusted_mode =
> -					&crtc_state->hw.adjusted_mode;
> -	struct intel_connector *connector = to_intel_connector(conn_state->connector);
> -	int symbol_size = intel_dp_is_uhbr(crtc_state) ? 32 : 8;
> -	/*
> -	 * min symbol cycles is 3(BS,VBID, BE) for 128b/132b and
> -	 * 5(BS, VBID, MVID, MAUD, BE) for 8b/10b
> -	 */
> -	int min_sym_cycles = intel_dp_is_uhbr(crtc_state) ? 3 : 5;
>  	bool is_mst = intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DP_MST);
> -	int num_joined_pipes = intel_crtc_num_joined_pipes(crtc_state);
> -	int min_hblank;
> -	int max_lane_count = 4;
> -	int hactive_sym_cycles, htotal_sym_cycles;
> -	int dsc_slices = 0;
> -	int link_bpp_x16;
>  
>  	if (DISPLAY_VER(display) < 30)
>  		return 0;
> @@ -3139,45 +3124,18 @@ int intel_dp_compute_min_hblank(struct intel_crtc_state *crtc_state,
>  	if (!is_mst && !intel_dp_is_uhbr(crtc_state))
>  		return 0;
>  
> -	if (crtc_state->dsc.compression_enable) {
> -		dsc_slices = intel_dp_dsc_get_slice_count(connector,
> -							  adjusted_mode->crtc_clock,
> -							  adjusted_mode->crtc_hdisplay,
> -							  num_joined_pipes);
> -		if (!dsc_slices) {
> -			drm_dbg(display->drm, "failed to calculate dsc slice count\n");
> -			return -EINVAL;
> -		}
> +	/* min_hblank formula is undergoing a change and the recomendation is to use static value */
> +	if (intel_dp_is_uhbr(crtc_state)) {
> +		if (crtc_state->dsc.compression_enable &&
> +		    crtc_state->output_format == INTEL_OUTPUT_FORMAT_YCBCR420 &&
> +		    crtc_state->pipe_bpp == 18)
> +			crtc_state->min_hblank = 2;
> +		else
> +			crtc_state->min_hblank = 3;
> +	} else {
> +		crtc_state->min_hblank = 10;
>  	}

The min hblank programmed can't be higher than the hblank period of the
mode. So the hblank symbol cycles for the mode should be still calculated
as now and the above new limit of 2/3 cycles for UHBR should be used
as needed for the upper bound of min_hblank below at

	min_hblank = min(10, min_hblank);

instead of the fixed limit of 10 cycles.

>  
> -	if (crtc_state->dsc.compression_enable)
> -		link_bpp_x16 = crtc_state->dsc.compressed_bpp_x16;
> -	else
> -		link_bpp_x16 = fxp_q4_from_int(intel_dp_output_bpp(crtc_state->output_format,
> -								   crtc_state->pipe_bpp));
> -
> -	/* Calculate min Hblank Link Layer Symbol Cycle Count for 8b/10b MST & 128b/132b */
> -	hactive_sym_cycles = drm_dp_link_symbol_cycles(max_lane_count,
> -						       adjusted_mode->hdisplay,
> -						       dsc_slices,
> -						       link_bpp_x16,
> -						       symbol_size, is_mst);
> -	htotal_sym_cycles = adjusted_mode->htotal * hactive_sym_cycles /
> -			     adjusted_mode->hdisplay;
> -
> -	min_hblank = htotal_sym_cycles - hactive_sym_cycles;
> -	/* minimum Hblank calculation: https://groups.vesa.org/wg/DP/document/20494 */
> -	min_hblank = max(min_hblank, min_sym_cycles);
> -
> -	/*
> -	 * adjust the BlankingStart/BlankingEnd framing control from
> -	 * the calculated value
> -	 */
> -	min_hblank = min_hblank - 2;
> -
> -	min_hblank = min(10, min_hblank);
> -	crtc_state->min_hblank = min_hblank;
> -
>  	return 0;
>  }
>  
> 
> ---
> base-commit: da56936fad6f5e1d5f0cef8b50277bfb071eefe7
> change-id: 20250624-min_hblank-8af8d2626ff8
> 
> Best regards,
> -- 
> Arun R Murthy <arun.r.murthy at intel.com>
>