[Intel-gfx] [PATCH v3] drm/i915/dp: Use max params for panels < eDP 1.4

[Kai-Heng Feng]

Users reported that after commit 2bbd6dba84d4 ("drm/i915: Try to use
fast+narrow link on eDP again and fall back to the old max strategy on
failure"), the screen starts to have wobbly effect.

Commit a5c936add6a2 ("drm/i915/dp: Use slow and wide link training for
everything") doesn't help either, that means the affected eDP 1.2 panels
only work with max params.

So use max params for panels < eDP 1.4 as Windows does to solve the
issue.

v3:
 - Do the eDP rev check in intel_edp_init_dpcd()

v2:
 - Check eDP 1.4 instead of DPCD 1.1 to apply max params

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/3714
Fixes: 2bbd6dba84d4 ("drm/i915: Try to use fast+narrow link on eDP again and fall back to the old max strategy on failure")
Fixes: a5c936add6a2 ("drm/i915/dp: Use slow and wide link training for everything")
Suggested-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
Signed-off-by: Kai-Heng Feng <kai.heng.feng at canonical.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 75d4ebc669411..e0dbd35ae7bc0 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -2445,11 +2445,14 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
 	 */
 	if (drm_dp_dpcd_read(&intel_dp->aux, DP_EDP_DPCD_REV,
 			     intel_dp->edp_dpcd, sizeof(intel_dp->edp_dpcd)) ==
-			     sizeof(intel_dp->edp_dpcd))
+			     sizeof(intel_dp->edp_dpcd)) {
 		drm_dbg_kms(&dev_priv->drm, "eDP DPCD: %*ph\n",
 			    (int)sizeof(intel_dp->edp_dpcd),
 			    intel_dp->edp_dpcd);
 
+		intel_dp->use_max_params = intel_dp->edp_dpcd[0] < DP_EDP_14;
+	}
+
 	/*
 	 * This has to be called after intel_dp->edp_dpcd is filled, PSR checks
 	 * for SET_POWER_CAPABLE bit in intel_dp->edp_dpcd[1]
-- 
2.32.0