drm/i915/hdcp: Read Rxcaps for robustibility

We see some monitors and docks report incorrect hdcp version
and capability in first few reads so we read rx_caps three times
before we conclude the monitor's or docks HDCP capability

--v2
-Add comment to justify the 3 time read loop for hdcp capability[Ankit]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240226063051.1685326-7-suraj.kandpal@intel.com

diff --git a/drivers/gpu/drm/i915/display/intel_dp_hdcp.c b/drivers/gpu/drm/i915/display/intel_dp_hdcp.c
index 99220f9..b98a878 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_dp_hdcp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_hdcp.c
@@ -644,18 +644,29 @@ int _intel_dp_hdcp2_get_capability(struct drm_dp_aux *aux,
                                   bool *capable)
 {
        u8 rx_caps[3];
-       int ret;
+       int ret, i;
 
        *capable = false;
-       ret = drm_dp_dpcd_read(aux,
-                              DP_HDCP_2_2_REG_RX_CAPS_OFFSET,
-                              rx_caps, HDCP_2_2_RXCAPS_LEN);
-       if (ret != HDCP_2_2_RXCAPS_LEN)
-               return ret >= 0 ? -EIO : ret;
 
-       if (rx_caps[0] == HDCP_2_2_RX_CAPS_VERSION_VAL &&
-           HDCP_2_2_DP_HDCP_CAPABLE(rx_caps[2]))
-               *capable = true;
+       /*
+        * Some HDCP monitors act really shady by not giving the correct hdcp
+        * capability on the first rx_caps read and usually take an extra read
+        * to give the capability. We read rx_caps three times before we
+        * declare a monitor not capable of HDCP 2.2.
+        */
+       for (i = 0; i < 3; i++) {
+               ret = drm_dp_dpcd_read(aux,
+                                      DP_HDCP_2_2_REG_RX_CAPS_OFFSET,
+                                      rx_caps, HDCP_2_2_RXCAPS_LEN);
+               if (ret != HDCP_2_2_RXCAPS_LEN)
+                       return ret >= 0 ? -EIO : ret;
+
+               if (rx_caps[0] == HDCP_2_2_RX_CAPS_VERSION_VAL &&
+                   HDCP_2_2_DP_HDCP_CAPABLE(rx_caps[2])) {
+                       *capable = true;
+                       break;
+               }
+       }
 
        return 0;
 }