drm/i915/dp: Improve clock recovery loop limit comment

Clarifies the clock recovery loop limit comment that 80
max_cr_tries for pre-DP1.4 devices was chosen as a very
tolerant upper bound.
Assumptions made:
- DP1.4 syncs should be smarter so they won't need more
than 10 tries
- pre-DP1.4 syncs should be compliant enough to not need
that many tries (80) but we should tolerate any that may
trigger this corner case

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Marc Herbert <marc.herbert@intel.com>
Suggested-by: Marc Herbert <marc.herbert@intel.com>
Signed-off-by: Nathan Ciobanu <nathan.d.ciobanu@linux.intel.com>
Reviewed-by: Marc Herbert <marc.herbert@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1532471612-30001-1-git-send-email-nathan.d.ciobanu@linux.intel.com

diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c b/drivers/gpu/drm/i915/intel_dp_link_training.c
index 07e128c..a9f4098 100644 (file)
--- a/drivers/gpu/drm/i915/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
@@ -172,10 +172,12 @@ intel_dp_link_training_clock_recovery(struct intel_dp *intel_dp)
        }
 
        /*
-        * 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)
+        * The DP 1.4 spec defines the max clock recovery retries value
+        * as 10 but for pre-DP 1.4 devices we set a very tolerant
+        * retry limit of 80 (4 voltage levels x 4 preemphasis levels x
+        * x 5 identical voltage retries). Since the previous specs didn't
+        * define a limit and created the possibility of an infinite loop
+        * we want to prevent any sync from triggering that corner case.
         */
        if (intel_dp->dpcd[DP_DPCD_REV] >= DP_DPCD_REV_14)
                max_cr_tries = 10;