drm/i915: implement EXTENDED_RECEIVER_CAPABILITY_FIELD_PRESENT

According to DP spec (2.9.3.1 of DP 1.4) if
EXTENDED_RECEIVER_CAPABILITY_FIELD_PRESENT is set the addresses in DPCD
02200h through 0220Fh shall contain the DPRX's true capability. These
values will match 00000h through 0000Fh, except for DPCD_REV,
MAX_LINK_RATE, DOWN_STREAM_PORT_PRESENT.

Read from DPCD once for all 3 values as this is an expensive operation.
Spec mentions that all of address space 02200h through 0220Fh should
contain the right information however currently only 3 values can
differ.

There is no address space in the intel_dp->dpcd struct for addresses
02200h through 0220Fh, and since so much of the data is a identical,
simply overwrite the values stored in 00000h through 0000Fh with the
values that can be overwritten from addresses 02200h through 0220Fh.

This patch helps with backward compatibility for devices pre DP1.3.

v2: read only dpcd values which can be affected, remove incorrect check,
split into drm include changes into separate patch, commit message,
verbose debugging statements during overwrite.
v3: white space fixes
v4: make path dependent on DPCD revision > 1.2
v5: split into function, removed DPCD rev check
v6: add debugging prints for early exit conditions
v7 (From Manasi):
* Memcpy, memcmp and debig logging based on sizeof(dpcd_ext) (Jani N)
* Exit early (Jani N)
v8 (From Manasi):
* Get rid of superfluous debug prints (Jani N)
* Print entire base DPCD before memcpy (Jani N)
v9 (From Manasi):
* Add uniform newlines (Rodrigo)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Tested-by: Manasi Navare <manasi.d.navare@intel.com>
Acked-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129220058.19636-1-manasi.d.navare@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -3991,6 +3991,42 @@ intel_dp_link_down(struct intel_encoder *encoder,
 	}
 }
 
+static void
+intel_dp_extended_receiver_capabilities(struct intel_dp *intel_dp)
+{
+	u8 dpcd_ext[6];
+
+	/*
+	 * Prior to DP1.3 the bit represented by
+	 * DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT was reserved.
+	 * if it is set DP_DPCD_REV at 0000h could be at a value less than
+	 * the true capability of the panel. The only way to check is to
+	 * then compare 0000h and 2200h.
+	 */
+	if (!(intel_dp->dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
+	      DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT))
+		return;
+
+	if (drm_dp_dpcd_read(&intel_dp->aux, DP_DP13_DPCD_REV,
+			     &dpcd_ext, sizeof(dpcd_ext)) != sizeof(dpcd_ext)) {
+		DRM_ERROR("DPCD failed read at extended capabilities\n");
+		return;
+	}
+
+	if (intel_dp->dpcd[DP_DPCD_REV] > dpcd_ext[DP_DPCD_REV]) {
+		DRM_DEBUG_KMS("DPCD extended DPCD rev less than base DPCD rev\n");
+		return;
+	}
+
+	if (!memcmp(intel_dp->dpcd, dpcd_ext, sizeof(dpcd_ext)))
+		return;
+
+	DRM_DEBUG_KMS("Base DPCD: %*ph\n",
+		      (int)sizeof(intel_dp->dpcd), intel_dp->dpcd);
+
+	memcpy(intel_dp->dpcd, dpcd_ext, sizeof(dpcd_ext));
+}
+
 bool
 intel_dp_read_dpcd(struct intel_dp *intel_dp)
 {
@@ -3998,6 +4034,8 @@ intel_dp_read_dpcd(struct intel_dp *intel_dp)
 			     sizeof(intel_dp->dpcd)) < 0)
 		return false; /* aux transfer failed */
 
+	intel_dp_extended_receiver_capabilities(intel_dp);
+
 	DRM_DEBUG_KMS("DPCD: %*ph\n", (int) sizeof(intel_dp->dpcd), intel_dp->dpcd);
 
 	return intel_dp->dpcd[DP_DPCD_REV] != 0;