drm/i915/dp: Limit link training clock recovery loop

Limit the link training clock recovery loop to 10 attempts at
LANEx_CR_DONE per DP 1.4 spec section 3.5.1.2.2 and 80 attempts for
pre-DP 1.4 (4 voltage levels x 4 preemphasis levels x
x 5 identical voltages tries). Some faulty USB-C MST hubs can
cause us to get stuck in this loop indefinitely requesting something
like:

    voltage swing: 0, pre-emphasis level: 2
    voltage swing: 1, pre-emphasis level: 2
    voltage swing: 0, pre-emphasis level: 3

over and over so max_vswing would never be reached,
drm_dp_clock_recovery_ok() would never return true and voltage_tries
would always get reset to 1. The driver sends those values to the hub
but the hub keeps requesting new values every time.

Changes in v2:
    - updated commit message (DK, Manasi)
    - defined DP_DP14_MAX_CR_TRIES (Marc)
    - made the loop iterate for max 10 times (Rodrigo, Marc)

Changes in v3:
    - changed error message to use DP_DP14_MAX_CR_TRIES

Changes in v4:
    - Updated the title to reflect the change
    - Updated the commit message
    - Added 80 attempts for pre-DP 1.4 devices

Changes in v5:
    - Removed DP_DP14_MAX_CR_TRIES from drm

v6: Updated comment to match kernel style (Rodrigo)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Marc Herbert <marc.herbert@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Nathan Ciobanu <nathan.d.ciobanu@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180720214413.29506-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_dp_link_training.c

@@ -129,7 +129,7 @@ static bool
 intel_dp_link_training_clock_recovery(struct intel_dp *intel_dp)
 {
 	uint8_t voltage;
-	int voltage_tries, max_vswing_tries;
+	int voltage_tries, max_vswing_tries, cr_tries, max_cr_tries;
 	uint8_t link_config[2];
 	uint8_t link_bw, rate_select;
 
@@ -170,9 +170,20 @@ intel_dp_link_training_clock_recovery(struct intel_dp *intel_dp)
 		return false;
 	}
 
+	/*
+	 * DP 1.4 spec clock recovery retries defined but
+	 * for devices pre-DP 1.4 we set the retry limit
+	 * to 4 (voltage levels) x 4 (preemphasis levels) x
+	 * x 5 (same voltage retries) = 80 (max iterations)
+	 */
+	if (intel_dp->dpcd[DP_DPCD_REV] >= DP_DPCD_REV_14)
+		max_cr_tries = 10;
+	else
+		max_cr_tries = 80;
+
 	voltage_tries = 1;
 	max_vswing_tries = 0;
-	for (;;) {
+	for (cr_tries = 0; cr_tries < max_cr_tries; ++cr_tries) {
 		uint8_t link_status[DP_LINK_STATUS_SIZE];
 
 		drm_dp_link_train_clock_recovery_delay(intel_dp->dpcd);
@@ -216,6 +227,8 @@ intel_dp_link_training_clock_recovery(struct intel_dp *intel_dp)
 			++max_vswing_tries;
 
 	}
+	DRM_ERROR("Failed clock recovery %d times, giving up!\n", max_cr_tries);
+	return false;
 }
 
 /*