drm/i915/dp: Avoid forcing DSC BPC for MST case

For MST the bpc is hardcoded to 8, and pipe bpp to 24.
So avoid forcing DSC bpc for MST case.

v2: Warn and ignore the debug flag than to bail out. (Jani)

v3: Fix dbg message to mention forced bpc instead of bpp.

v4: Fix checkpatch longline warning.
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230817142459.89764-9-ankit.k.nautiyal@intel.com

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

@@ -1697,14 +1697,14 @@ int intel_dp_dsc_compute_config(struct intel_dp *intel_dp,
 	if (!intel_dp_dsc_supports_format(intel_dp, pipe_config->output_format))
 		return -EINVAL;
 
-	if (compute_pipe_bpp)
+	if (intel_dp->force_dsc_bpc && compute_pipe_bpp) {
+		pipe_bpp = intel_dp->force_dsc_bpc * 3;
+		drm_dbg_kms(&dev_priv->drm, "Input DSC BPC forced to %d\n",
+			    intel_dp->force_dsc_bpc);
+	} else if (compute_pipe_bpp) {
 		pipe_bpp = intel_dp_dsc_compute_bpp(intel_dp, conn_state->max_requested_bpc);
-	else
+	} else {
 		pipe_bpp = pipe_config->pipe_bpp;
-
-	if (intel_dp->force_dsc_bpc) {
-		pipe_bpp = intel_dp->force_dsc_bpc * 3;
-		drm_dbg_kms(&dev_priv->drm, "Input DSC BPP forced to %d", pipe_bpp);
 	}
 
 	/* Min Input BPC for ICL+ is 8 */

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

@@ -361,6 +361,11 @@ static int intel_dp_mst_compute_config(struct intel_encoder *encoder,
 	/* enable compression if the mode doesn't fit available BW */
 	drm_dbg_kms(&dev_priv->drm, "Force DSC en = %d\n", intel_dp->force_dsc_en);
 	if (ret || intel_dp->force_dsc_en) {
+		/*
+		 * FIXME: As bpc is hardcoded to 8, as mentioned above,
+		 * WARN and ignore the debug flag force_dsc_bpc for now.
+		 */
+		drm_WARN(&dev_priv->drm, intel_dp->force_dsc_bpc, "Cannot Force BPC for MST\n");
 		/*
 		 * Try to get at least some timeslots and then see, if
 		 * we can fit there with DSC.