drm/i915/dp_mst: Account for channel coding efficiency in the DSC DPT bpp limit

The DSC DPT interface BW limit check should take into account the link
clock's (aka DDI clock in bspec) channel coding efficiency overhead.
Bspec suggests that the FEC overhead needs to be applied, however HW
people claim this isn't the case, nor is any overhead applicable.

However based on testing various 5k/6k modes both on the DELL U3224KBA
monitor and the Unigraf UCD-500 CTS test device, both the channel coding
efficiency (which includes the FEC overhead) and an additional 3%
overhead must be accounted for to get these modes working.

Bspec: 49259

v2:
- Apply an additional 3% overhead, add a commit log and code comment
  about these overheads and the relation to the Bspec BW limit formula.
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416221010.376865-5-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -59,11 +59,30 @@ static int intel_dp_mst_check_constraints(struct drm_i915_private *i915, int bpp
 	if (intel_dp_is_uhbr(crtc_state) && DISPLAY_VER(i915) < 14 && dsc) {
 		int output_bpp = bpp;
 		int symbol_clock = intel_dp_link_symbol_clock(crtc_state->port_clock);
+		/*
+		 * Bspec/49259 suggests that the FEC overhead needs to be
+		 * applied here, though HW people claim that neither this FEC
+		 * or any other overhead is applicable here (that is the actual
+		 * available_bw is just symbol_clock * 72). However based on
+		 * testing on MTL-P the
+		 * - DELL U3224KBA display
+		 * - Unigraf UCD-500 CTS test sink
+		 * devices the
+		 * - 5120x2880/995.59Mhz
+		 * - 6016x3384/1357.23Mhz
+		 * - 6144x3456/1413.39Mhz
+		 * modes (all the ones having a DPT limit on the above devices),
+		 * both the channel coding efficiency and an additional 3%
+		 * overhead needs to be accounted for.
+		 */
+		int available_bw = mul_u32_u32(symbol_clock * 72,
+					       drm_dp_bw_channel_coding_efficiency(true)) /
+				   1030000;
 
 		if (output_bpp * adjusted_mode->crtc_clock >
-		    symbol_clock * 72) {
+		    available_bw) {
 			drm_dbg_kms(&i915->drm, "UHBR check failed(required bw %d available %d)\n",
-				    output_bpp * adjusted_mode->crtc_clock, symbol_clock * 72);
+				    output_bpp * adjusted_mode->crtc_clock, available_bw);
 			return -EINVAL;
 		}
 	}