drm: Helper to read max clock rate

Helper routine to read out maximum supported pixel rate
for DisplayPort legay VGA converter or TMDS clock rate
for other digital legacy converters. The helper returns
clock rate in kHz.

v2: Return early if detailed port cap info is not available.
    Replace if-else ladder with switch-case (Ville)
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473419458-17080-4-git-send-email-mika.kahola@intel.com

drivers/gpu/drm/drm_dp_helper.c

@@ -438,6 +438,39 @@ int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link)
 }
 EXPORT_SYMBOL(drm_dp_link_configure);
 
+/**
+ * drm_dp_downstream_max_clock() - extract branch device max
+ *                                 pixel rate for legacy VGA
+ *                                 converter or max TMDS clock
+ *                                 rate for others
+ * @dpcd: DisplayPort configuration data
+ * @port_cap: port capabilities
+ *
+ * Returns max clock in kHz on success or 0 if max clock not defined
+ */
+int drm_dp_downstream_max_clock(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
+				const u8 port_cap[4])
+{
+	int type = port_cap[0] & DP_DS_PORT_TYPE_MASK;
+	bool detailed_cap_info = dpcd[DP_DOWNSTREAMPORT_PRESENT] &
+		DP_DETAILED_CAP_INFO_AVAILABLE;
+
+	if (!detailed_cap_info)
+		return 0;
+
+	switch (type) {
+	case DP_DS_PORT_TYPE_VGA:
+		return port_cap[1] * 8 * 1000;
+	case DP_DS_PORT_TYPE_DVI:
+	case DP_DS_PORT_TYPE_HDMI:
+	case DP_DS_PORT_TYPE_DP_DUALMODE:
+		return port_cap[1] * 2500;
+	default:
+		return 0;
+	}
+}
+EXPORT_SYMBOL(drm_dp_downstream_max_clock);
+
 /*
  * I2C-over-AUX implementation
  */

include/drm/drm_dp_helper.h

@@ -815,6 +815,8 @@ int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link);
 int drm_dp_link_power_up(struct drm_dp_aux *aux, struct drm_dp_link *link);
 int drm_dp_link_power_down(struct drm_dp_aux *aux, struct drm_dp_link *link);
 int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link);
+int drm_dp_downstream_max_clock(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
+				const u8 port_cap[4]);
 
 void drm_dp_aux_init(struct drm_dp_aux *aux);
 int drm_dp_aux_register(struct drm_dp_aux *aux);