[Intel-gfx] [PATCH v2] drm/i915/dp: Use max params for panels < eDP 1.4

[Kai-Heng Feng <kai.heng.feng@canonical.com>]

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.

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@linux.intel.com>
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 75d4ebc669411..f87fad78f1a9f 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -1330,14 +1330,16 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
 	limits.min_bpp = intel_dp_min_bpp(pipe_config->output_format);
 	limits.max_bpp = intel_dp_max_bpp(intel_dp, pipe_config);
 
-	if (intel_dp->use_max_params) {
+	if (intel_dp->use_max_params ||
+	    intel_dp->edp_dpcd[0] < DP_EDP_14) {
 		/*
 		 * Use the maximum clock and number of lanes the eDP panel
 		 * advertizes being capable of in case the initial fast
-		 * optimal params failed us. The panels are generally
-		 * designed to support only a single clock and lane
-		 * configuration, and typically on older panels these
-		 * values correspond to the native resolution of the panel.
+		 * optimal params failed us or the EDP rev is earlier than 1.4.
+		 * The panels are generally designed to support only a single
+		 * clock and lane configuration, and typically on older panels
+		 * these values correspond to the native resolution of the
+		 * panel.
 		 */
 		limits.min_lane_count = limits.max_lane_count;
 		limits.min_clock = limits.max_clock;
-- 
2.32.0