drm/amdgpu: fix dp link rate selection (v2)

Need to properly handle the max link rate in the dpcd.
This prevents some cases where 5.4 Ghz is selected when
it shouldn't be.

v2: simplify logic, add array bounds check

Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/atombios_dp.c

@@ -243,7 +243,7 @@ static void amdgpu_atombios_dp_get_adjust_train(const u8 link_status[DP_LINK_STA
 
 /* convert bits per color to bits per pixel */
 /* get bpc from the EDID */
-static int amdgpu_atombios_dp_convert_bpc_to_bpp(int bpc)
+static unsigned amdgpu_atombios_dp_convert_bpc_to_bpp(int bpc)
 {
 	if (bpc == 0)
 		return 24;
@@ -251,64 +251,32 @@ static int amdgpu_atombios_dp_convert_bpc_to_bpp(int bpc)
 		return bpc * 3;
 }
 
-/* get the max pix clock supported by the link rate and lane num */
-static int amdgpu_atombios_dp_get_max_dp_pix_clock(int link_rate,
-					    int lane_num,
-					    int bpp)
-{
-	return (link_rate * lane_num * 8) / bpp;
-}
-
 /***** amdgpu specific DP functions *****/
 
-/* First get the min lane# when low rate is used according to pixel clock
- * (prefer low rate), second check max lane# supported by DP panel,
- * if the max lane# < low rate lane# then use max lane# instead.
- */
-static int amdgpu_atombios_dp_get_dp_lane_number(struct drm_connector *connector,
+static int amdgpu_atombios_dp_get_dp_link_config(struct drm_connector *connector,
 						 const u8 dpcd[DP_DPCD_SIZE],
-						 int pix_clock)
-{
-	int bpp = amdgpu_atombios_dp_convert_bpc_to_bpp(amdgpu_connector_get_monitor_bpc(connector));
-	int max_link_rate = drm_dp_max_link_rate(dpcd);
-	int max_lane_num = drm_dp_max_lane_count(dpcd);
-	int lane_num;
-	int max_dp_pix_clock;
-
-	for (lane_num = 1; lane_num < max_lane_num; lane_num <<= 1) {
-		max_dp_pix_clock = amdgpu_atombios_dp_get_max_dp_pix_clock(max_link_rate, lane_num, bpp);
-		if (pix_clock <= max_dp_pix_clock)
-			break;
-	}
-
-	return lane_num;
-}
-
-static int amdgpu_atombios_dp_get_dp_link_clock(struct drm_connector *connector,
-						const u8 dpcd[DP_DPCD_SIZE],
-						int pix_clock)
+						 unsigned pix_clock,
+						 unsigned *dp_lanes, unsigned *dp_rate)
 {
-	int bpp = amdgpu_atombios_dp_convert_bpc_to_bpp(amdgpu_connector_get_monitor_bpc(connector));
-	int lane_num, max_pix_clock;
-
-	if (amdgpu_connector_encoder_get_dp_bridge_encoder_id(connector) ==
-	    ENCODER_OBJECT_ID_NUTMEG)
-		return 270000;
-
-	lane_num = amdgpu_atombios_dp_get_dp_lane_number(connector, dpcd, pix_clock);
-	max_pix_clock = amdgpu_atombios_dp_get_max_dp_pix_clock(162000, lane_num, bpp);
-	if (pix_clock <= max_pix_clock)
-		return 162000;
-	max_pix_clock = amdgpu_atombios_dp_get_max_dp_pix_clock(270000, lane_num, bpp);
-	if (pix_clock <= max_pix_clock)
-		return 270000;
-	if (amdgpu_connector_is_dp12_capable(connector)) {
-		max_pix_clock = amdgpu_atombios_dp_get_max_dp_pix_clock(540000, lane_num, bpp);
-		if (pix_clock <= max_pix_clock)
-			return 540000;
+	unsigned bpp =
+		amdgpu_atombios_dp_convert_bpc_to_bpp(amdgpu_connector_get_monitor_bpc(connector));
+	static const unsigned link_rates[3] = { 162000, 270000, 540000 };
+	unsigned max_link_rate = drm_dp_max_link_rate(dpcd);
+	unsigned max_lane_num = drm_dp_max_lane_count(dpcd);
+	unsigned lane_num, i, max_pix_clock;
+
+	for (lane_num = 1; lane_num <= max_lane_num; lane_num <<= 1) {
+		for (i = 0; i < ARRAY_SIZE(link_rates) && link_rates[i] <= max_link_rate; i++) {
+			max_pix_clock = (lane_num * link_rates[i] * 8) / bpp;
+			if (max_pix_clock >= pix_clock) {
+				*dp_lanes = lane_num;
+				*dp_rate = link_rates[i];
+				return 0;
+			}
+		}
 	}
 
-	return drm_dp_max_link_rate(dpcd);
+	return -EINVAL;
 }
 
 static u8 amdgpu_atombios_dp_encoder_service(struct amdgpu_device *adev,
@@ -422,6 +390,7 @@ void amdgpu_atombios_dp_set_link_config(struct drm_connector *connector,
 {
 	struct amdgpu_connector *amdgpu_connector = to_amdgpu_connector(connector);
 	struct amdgpu_connector_atom_dig *dig_connector;
+	int ret;
 
 	if (!amdgpu_connector->con_priv)
 		return;
@@ -429,10 +398,14 @@ void amdgpu_atombios_dp_set_link_config(struct drm_connector *connector,
 
 	if ((dig_connector->dp_sink_type == CONNECTOR_OBJECT_ID_DISPLAYPORT) ||
 	    (dig_connector->dp_sink_type == CONNECTOR_OBJECT_ID_eDP)) {
-		dig_connector->dp_clock =
-			amdgpu_atombios_dp_get_dp_link_clock(connector, dig_connector->dpcd, mode->clock);
-		dig_connector->dp_lane_count =
-			amdgpu_atombios_dp_get_dp_lane_number(connector, dig_connector->dpcd, mode->clock);
+		ret = amdgpu_atombios_dp_get_dp_link_config(connector, dig_connector->dpcd,
+							    mode->clock,
+							    &dig_connector->dp_lane_count,
+							    &dig_connector->dp_clock);
+		if (ret) {
+			dig_connector->dp_clock = 0;
+			dig_connector->dp_lane_count = 0;
+		}
 	}
 }
 
@@ -441,14 +414,17 @@ int amdgpu_atombios_dp_mode_valid_helper(struct drm_connector *connector,
 {
 	struct amdgpu_connector *amdgpu_connector = to_amdgpu_connector(connector);
 	struct amdgpu_connector_atom_dig *dig_connector;
-	int dp_clock;
+	unsigned dp_lanes, dp_clock;
+	int ret;
 
 	if (!amdgpu_connector->con_priv)
 		return MODE_CLOCK_HIGH;
 	dig_connector = amdgpu_connector->con_priv;
 
-	dp_clock =
-		amdgpu_atombios_dp_get_dp_link_clock(connector, dig_connector->dpcd, mode->clock);
+	ret = amdgpu_atombios_dp_get_dp_link_config(connector, dig_connector->dpcd,
+						    mode->clock, &dp_lanes, &dp_clock);
+	if (ret)
+		return MODE_CLOCK_HIGH;
 
 	if ((dp_clock == 540000) &&
 	    (!amdgpu_connector_is_dp12_capable(connector)))