drm/i915/dp: add lane_count check in intel_dp_check_link_status

Currently it's entirely possible to go through the link training step
without first determining the lane_count, which is silly since we end up
doing a bunch of aux transfers of size = 0, as highlighted by
WARN_ON(!msg->buffer != !msg->size), and can only ever result in a
'failed to update link training' message. This can be observed during
intel_dp_long_pulse where we can do the link training step, but before
we have had a chance to set the link params. To avoid this we add an
extra check for the lane_count in intel_dp_check_link_status, which
should prevent us from doing the link training step prematurely.

v2: add WARN_ON_ONCE and FIXME comment (Ville)

References: https://bugs.freedesktop.org/show_bug.cgi?id=97344
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476912593-10019-1-git-send-email-matthew.auld@intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index e03e79f7714a6ddd6e163344b2bdc53aefca5663..f30db8f2425e43e8b23f6b6912de400ce2fd0a6c 100644 (file)
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -4041,6 +4041,11 @@ intel_dp_check_link_status(struct intel_dp *intel_dp)
        if (!to_intel_crtc(intel_encoder->base.crtc)->active)
                return;
 
+       /* FIXME: we need to synchronize this sort of stuff with hardware
+        * readout */
+       if (WARN_ON_ONCE(!intel_dp->lane_count))
+               return;
+
        /* if link training is requested we should perform it always */
        if ((intel_dp->compliance_test_type == DP_TEST_LINK_TRAINING) ||
            (!drm_dp_channel_eq_ok(link_status, intel_dp->lane_count))) {