drm/dp: Add DisplayPort link helpers

Add a helper to probe a DP link (read out the supported DPCD revision,
maximum rate, link count and capabilities) as well as power up the DP
link and configure it accordingly.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
---
Changes in v5:
- export helpers

Changes in v4:
- fix a couple of typos in comments as pointed out by Alex Deucher

Changes in v3:
- split into drm_dp_link_power_up() and drm_dp_link_configure()
- do not change sink state for DPCD versions earlier than 1.1
- sleep for 1-2 ms after setting local sink to D0 state
- read and write consecutive registers where possible
- read DPCD revision when link is probed
- remove duplicate kerneldoc

include/drm/drm_dp_helper.h

@@ -291,6 +291,7 @@
 #define DP_SET_POWER                        0x600
 # define DP_SET_POWER_D0                    0x1
 # define DP_SET_POWER_D3                    0x2
+# define DP_SET_POWER_MASK                  0x3
 
 #define DP_PSR_ERROR_STATUS                 0x2006  /* XXX 1.2? */
 # define DP_PSR_LINK_CRC_ERROR              (1 << 0)
@@ -480,4 +481,20 @@ static inline ssize_t drm_dp_dpcd_writeb(struct drm_dp_aux *aux,
 int drm_dp_dpcd_read_link_status(struct drm_dp_aux *aux,
 				 u8 status[DP_LINK_STATUS_SIZE]);
 
+/*
+ * DisplayPort link
+ */
+#define DP_LINK_CAP_ENHANCED_FRAMING (1 << 0)
+
+struct drm_dp_link {
+	unsigned char revision;
+	unsigned int rate;
+	unsigned int num_lanes;
+	unsigned long capabilities;
+};
+
+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_configure(struct drm_dp_aux *aux, struct drm_dp_link *link);
+
 #endif /* _DRM_DP_HELPER_H_ */