drm/i915: Extract drm_dp_atomic_find_vcpi_slots cycle to separate function

We would be using almost same code to loop through bpps while calling
drm_dp_atomic_find_vcpi_slots - lets remove this duplication by
introducing a new function intel_dp_mst_find_vcpi_slots_for_bpp

v2: Fix pbn_div calculation - shouldn't matter if its DSC or not.
v3: FIx rebase conflict, constant_n no longer needed.
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101094222.22091-4-stanislav.lisovskiy@intel.com

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

@@ -45,10 +45,14 @@
 #include "intel_hotplug.h"
 #include "skl_scaler.h"
 
-static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
-					    struct intel_crtc_state *crtc_state,
-					    struct drm_connector_state *conn_state,
-					    struct link_config_limits *limits)
+static int intel_dp_mst_find_vcpi_slots_for_bpp(struct intel_encoder *encoder,
+						struct intel_crtc_state *crtc_state,
+						int max_bpp,
+						int min_bpp,
+						struct link_config_limits *limits,
+						struct drm_connector_state *conn_state,
+						int step,
+						bool dsc)
 {
 	struct drm_atomic_state *state = crtc_state->uapi.state;
 	struct intel_dp_mst_encoder *intel_mst = enc_to_mst(encoder);
@@ -72,18 +76,20 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 	// TODO: Handle pbn_div changes by adding a new MST helper
 	if (!mst_state->pbn_div) {
 		mst_state->pbn_div = drm_dp_get_vc_payload_bw(&intel_dp->mst_mgr,
-							      limits->max_rate,
-							      limits->max_lane_count);
+							      crtc_state->port_clock,
+							      crtc_state->lane_count);
 	}
 
-	for (bpp = limits->max_bpp; bpp >= limits->min_bpp; bpp -= 2 * 3) {
+	for (bpp = max_bpp; bpp >= min_bpp; bpp -= step) {
 		crtc_state->pipe_bpp = bpp;
 
 		crtc_state->pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock,
-						       crtc_state->pipe_bpp,
-						       false);
+						       dsc ? bpp << 4 : crtc_state->pipe_bpp,
+						       dsc);
+
 		slots = drm_dp_atomic_find_time_slots(state, &intel_dp->mst_mgr,
-						      connector->port, crtc_state->pbn);
+						      connector->port,
+						      crtc_state->pbn);
 		if (slots == -EDEADLK)
 			return slots;
 		if (slots >= 0) {
@@ -101,11 +107,29 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 	if (ret && slots >= 0)
 		slots = ret;
 
-	if (slots < 0) {
+	if (slots < 0)
 		drm_dbg_kms(&i915->drm, "failed finding vcpi slots:%d\n",
 			    slots);
+
+	return slots;
+}
+
+
+static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
+					    struct intel_crtc_state *crtc_state,
+					    struct drm_connector_state *conn_state,
+					    struct link_config_limits *limits)
+{
+	const struct drm_display_mode *adjusted_mode =
+		&crtc_state->hw.adjusted_mode;
+	int slots = -EINVAL;
+
+	slots = intel_dp_mst_find_vcpi_slots_for_bpp(encoder, crtc_state, limits->max_bpp,
+						     limits->min_bpp, limits,
+						     conn_state, 2 * 3, false);
+
+	if (slots < 0)
 		return slots;
-	}
 
 	intel_link_compute_m_n(crtc_state->pipe_bpp,
 			       crtc_state->lane_count,