drm/i915/dp: Add workaround for spurious AUX timeouts/hotplugs on LTTPR links

To avoid AUX timeouts and subsequent spurious hotplug interrupts, make
sure that the first DPCD access during detection is a read from an LTTPR
register.

Some ADLP DP link configuration at least with multiple LTTPRs expects
the first DPCD access during the LTTPR/DPCD detection after hotplug to
be a read from the LTTPR range starting with
DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV. The side effect of
this read is to put each LTTPR into the LTTPR transparent or LTTPR
non-transparent mode.

The lack of the above read may leave some of the LTTPRs in non-LTTPR
mode, while other LTTPRs in LTTPR transparent or LTTPR non-transparent
mode (for instance LTTPRs after system suspend/resume that kept their
mode from before suspend). Due to the different AUX timeouts the
different modes imply, the DPCD access from a non-LTTPR range will
timeout and lead to an LTTPR generated hotplug towards the source (which
the LTTPR firmware uses to account for buggy TypeC adapters with a long
wake-up delay).

SYSCROS: 72939

v2: Keep DPCD read-out working on non-LTTPR platforms.
v3: Summarize what and why the patch does at the beginning of the commit
    log. (Jani)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220408224629.845887-1-imre.deak@intel.com

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

@@ -82,19 +82,8 @@ static bool intel_dp_read_lttpr_common_caps(struct intel_dp *intel_dp,
 					    const u8 dpcd[DP_RECEIVER_CAP_SIZE])
 {
 	struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
-	struct drm_i915_private *i915 = to_i915(encoder->base.dev);
 	int ret;
 
-	if (intel_dp_is_edp(intel_dp))
-		return false;
-
-	/*
-	 * Detecting LTTPRs must be avoided on platforms with an AUX timeout
-	 * period < 3.2ms. (see DP Standard v2.0, 2.11.2, 3.6.6.1).
-	 */
-	if (DISPLAY_VER(i915) < 10 || IS_GEMINILAKE(i915))
-		return false;
-
 	ret = drm_dp_read_lttpr_common_caps(&intel_dp->aux, dpcd,
 					    intel_dp->lttpr_common_caps);
 	if (ret < 0)
@@ -197,13 +186,25 @@ static int intel_dp_init_lttpr(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEI
  */
 int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
 {
-	u8 dpcd[DP_RECEIVER_CAP_SIZE];
-	int lttpr_count;
+	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
+	int lttpr_count = 0;
 
-	if (drm_dp_read_dpcd_caps(&intel_dp->aux, dpcd))
-		return -EIO;
+	/*
+	 * Detecting LTTPRs must be avoided on platforms with an AUX timeout
+	 * period < 3.2ms. (see DP Standard v2.0, 2.11.2, 3.6.6.1).
+	 */
+	if (!intel_dp_is_edp(intel_dp) &&
+	    (DISPLAY_VER(i915) >= 10 && !IS_GEMINILAKE(i915))) {
+		u8 dpcd[DP_RECEIVER_CAP_SIZE];
+
+		if (drm_dp_dpcd_probe(&intel_dp->aux, DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV))
+			return -EIO;
 
-	lttpr_count = intel_dp_init_lttpr(intel_dp, dpcd);
+		if (drm_dp_read_dpcd_caps(&intel_dp->aux, dpcd))
+			return -EIO;
+
+		lttpr_count = intel_dp_init_lttpr(intel_dp, dpcd);
+	}
 
 	/*
 	 * The DPTX shall read the DPRX caps after LTTPR detection, so re-read