drm/i915/dp: Extract drm_dp_read_downstream_info()

We're going to be doing the same probing process in nouveau for
determining downstream DP port capabilities, so let's deduplicate the
work by moving i915's code for handling this into a shared helper:
drm_dp_read_downstream_info().

Note that when we do this, we also do make some functional changes while
we're at it:
* We always clear the downstream port info before trying to read it,
  just to make things easier for the caller
* We skip reading downstream port info if the DPCD indicates that we
  don't support downstream port info
* We only read as many bytes as needed for the reported number of
  downstream ports, no sense in reading the whole thing every time

v2:
* Fixup logic for calculating the downstream port length to account for
  the fact that downstream port caps can be either 1 byte or 4 bytes
  long. We can actually skip fixing the max_clock/max_bpc helpers here
  since they all check for DP_DETAILED_CAP_INFO_AVAILABLE anyway.
* Fix ret code check for drm_dp_dpcd_read
v5:
* Change name from drm_dp_downstream_read_info() to
  drm_dp_read_downstream_info()
* Also, add "See Also" sections for the various downstream info
  functions (drm_dp_read_downstream_info(), drm_dp_downstream_max_clock(),
  drm_dp_downstream_max_bpc())

Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-14-lyude@redhat.com

drivers/gpu/drm/drm_dp_helper.c

@@ -423,6 +423,56 @@ bool drm_dp_send_real_edid_checksum(struct drm_dp_aux *aux,
 }
 EXPORT_SYMBOL(drm_dp_send_real_edid_checksum);
 
+static u8 drm_dp_downstream_port_count(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
+{
+	u8 port_count = dpcd[DP_DOWN_STREAM_PORT_COUNT] & DP_PORT_COUNT_MASK;
+
+	if (dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE && port_count > 4)
+		port_count = 4;
+
+	return port_count;
+}
+
+/**
+ * drm_dp_read_downstream_info() - read DPCD downstream port info if available
+ * @aux: DisplayPort AUX channel
+ * @dpcd: A cached copy of the port's DPCD
+ * @downstream_ports: buffer to store the downstream port info in
+ *
+ * See also:
+ * drm_dp_downstream_max_clock()
+ * drm_dp_downstream_max_bpc()
+ *
+ * Returns: 0 if either the downstream port info was read successfully or
+ * there was no downstream info to read, or a negative error code otherwise.
+ */
+int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
+				const u8 dpcd[DP_RECEIVER_CAP_SIZE],
+				u8 downstream_ports[DP_MAX_DOWNSTREAM_PORTS])
+{
+	int ret;
+	u8 len;
+
+	memset(downstream_ports, 0, DP_MAX_DOWNSTREAM_PORTS);
+
+	/* No downstream info to read */
+	if (!drm_dp_is_branch(dpcd) ||
+	    dpcd[DP_DPCD_REV] < DP_DPCD_REV_10 ||
+	    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
+		return 0;
+
+	len = drm_dp_downstream_port_count(dpcd);
+	if (dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE)
+		len *= 4;
+
+	ret = drm_dp_dpcd_read(aux, DP_DOWNSTREAM_PORT_0, downstream_ports, len);
+	if (ret < 0)
+		return ret;
+
+	return ret == len ? 0 : -EIO;
+}
+EXPORT_SYMBOL(drm_dp_read_downstream_info);
+
 /**
  * drm_dp_downstream_max_clock() - extract branch device max
  *                                 pixel rate for legacy VGA
@@ -431,7 +481,11 @@ EXPORT_SYMBOL(drm_dp_send_real_edid_checksum);
  * @dpcd: DisplayPort configuration data
  * @port_cap: port capabilities
  *
- * Returns max clock in kHz on success or 0 if max clock not defined
+ * See also:
+ * drm_dp_read_downstream_info()
+ * drm_dp_downstream_max_bpc()
+ *
+ * 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])
@@ -462,7 +516,11 @@ EXPORT_SYMBOL(drm_dp_downstream_max_clock);
  * @dpcd: DisplayPort configuration data
  * @port_cap: port capabilities
  *
- * Returns max bpc on success or 0 if max bpc not defined
+ * See also:
+ * drm_dp_read_downstream_info()
+ * drm_dp_downstream_max_clock()
+ *
+ * Returns: Max bpc on success or 0 if max bpc not defined
  */
 int drm_dp_downstream_max_bpc(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
 			      const u8 port_cap[4])

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

@@ -4685,18 +4685,8 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
 			return false;
 	}
 
-	if (!drm_dp_is_branch(intel_dp->dpcd))
-		return true; /* native DP sink */
-
-	if (intel_dp->dpcd[DP_DPCD_REV] == 0x10)
-		return true; /* no per-port downstream info */
-
-	if (drm_dp_dpcd_read(&intel_dp->aux, DP_DOWNSTREAM_PORT_0,
-			     intel_dp->downstream_ports,
-			     DP_MAX_DOWNSTREAM_PORTS) < 0)
-		return false; /* downstream port status fetch failed */
-
-	return true;
+	return drm_dp_read_downstream_info(&intel_dp->aux, intel_dp->dpcd,
+					   intel_dp->downstream_ports) == 0;
 }
 
 static bool

include/drm/drm_dp_helper.h

@@ -1613,6 +1613,9 @@ int drm_dp_dpcd_read_link_status(struct drm_dp_aux *aux,
 bool drm_dp_send_real_edid_checksum(struct drm_dp_aux *aux,
 				    u8 real_edid_checksum);
 
+int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
+				const u8 dpcd[DP_RECEIVER_CAP_SIZE],
+				u8 downstream_ports[DP_MAX_DOWNSTREAM_PORTS]);
 int drm_dp_downstream_max_clock(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
 				const u8 port_cap[4]);
 int drm_dp_downstream_max_bpc(const u8 dpcd[DP_RECEIVER_CAP_SIZE],