[PATCH v2] drm/i915/dp: Give up link training clock recovery after 10 failed attempts

[Nathan Ciobanu <nathan.d.ciobanu@xxxxxxxxxxxxxxx>]

Limit the link training clock recovery loop to 10 failed attempts at
LANEx_CR_DONE per DP 1.4 spec section 3.5.1.2.2. Some USB-C MST hubs
cause us to get stuck in this loop indefinitely requesting

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

over and over: max_vswing is never reached, drm_dp_clock_recovery_ok()
never returns true and voltage_tries always gets reset to 1. The driver
sends those values to the hub but the hub keeps requesting new values
every time in the pattern shown above.

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)

Signed-off-by: Nathan Ciobanu <nathan.d.ciobanu@xxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/intel_dp_link_training.c | 6 ++++--
 include/drm/drm_dp_helper.h                   | 1 +
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c b/drivers/gpu/drm/i915/intel_dp_link_training.c
index 4da6e33c7fa1..671de62aacc2 100644
--- a/drivers/gpu/drm/i915/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
@@ -129,7 +129,7 @@ static bool intel_dp_link_max_vswing_reached(struct intel_dp *intel_dp)
 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;
 	uint8_t link_config[2];
 	uint8_t link_bw, rate_select;
 
@@ -172,7 +172,7 @@ static bool intel_dp_link_max_vswing_reached(struct intel_dp *intel_dp)
 
 	voltage_tries = 1;
 	max_vswing_tries = 0;
-	for (;;) {
+	for (cr_tries = 0; cr_tries < DP_DP14_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 +216,8 @@ static bool intel_dp_link_max_vswing_reached(struct intel_dp *intel_dp)
 			++max_vswing_tries;
 
 	}
+	DRM_ERROR("Failed clock recovery 10 times, giving up!\n");
+	return false;
 }
 
 /*
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index c01564991a9f..2303ad8ed24e 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -820,6 +820,7 @@
 
 #define DP_DP13_DPCD_REV                    0x2200
 #define DP_DP13_MAX_LINK_RATE               0x2201
+#define DP_DP14_MAX_CR_TRIES                10
 
 #define DP_DPRX_FEATURE_ENUMERATION_LIST    0x2210  /* DP 1.3 */
 # define DP_GTC_CAP					(1 << 0)  /* DP 1.3 */
-- 
1.9.1

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