[PATCH] drm/i915/ilk-glk: Fix link training on links with LTTPRs

[Imre Deak <imre.deak@intel.com>]

The spec requires to use at least 3.2ms for the AUX timeout period if
there are LT-tunable PHY Repeaters on the link (2.11.2). An upcoming
spec update makes this more specific, by requiring a 3.2ms minimum
timeout period for the LTTPR detection reading the 0xF0000-0xF0007
range (3.6.5.1).

Accordingly disable LTTPR detection until GLK, where the maximum timeout
we can set is only 1.6ms.

Link training in the non-transparent mode is known to fail at least on
some SKL systems with a WD19 dock on the link, which exposes an LTTPR
(see the References below). While this could have different reasons
besides the too short AUX timeout used, not detecting LTTPRs (and so not
using the non-transparent LT mode) fixes link training on these systems.

While at it add a code comment about the platform specific maximum
timeout values.

Reported-by: Takashi Iwai <tiwai@suse.de>
References: https://gitlab.freedesktop.org/drm/intel/-/issues/3166
Fixes: b30edfd8d0b4 ("drm/i915: Switch to LTTPR non-transparent mode link training")
Cc: <stable@vger.kernel.org> # v5.11
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp_aux.c           | 7 +++++++
 drivers/gpu/drm/i915/display/intel_dp_link_training.c | 8 ++++++++
 2 files changed, 15 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
index eaebf123310a..b581e8acce07 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
@@ -133,6 +133,7 @@ static u32 g4x_get_aux_send_ctl(struct intel_dp *intel_dp,
 	else
 		precharge = 5;
 
+	/* Max timeout value on ILK-BDW: 1.6ms */
 	if (IS_BROADWELL(dev_priv))
 		timeout = DP_AUX_CH_CTL_TIME_OUT_600us;
 	else
@@ -159,6 +160,12 @@ static u32 skl_get_aux_send_ctl(struct intel_dp *intel_dp,
 	enum phy phy = intel_port_to_phy(i915, dig_port->base.port);
 	u32 ret;
 
+	/*
+	 * Max timeout values:
+	 * SKL-GLK: 1.6ms
+	 * CNL: 3.2ms
+	 * ICL+: 4ms
+	 */
 	ret = DP_AUX_CH_CTL_SEND_BUSY |
 	      DP_AUX_CH_CTL_DONE |
 	      DP_AUX_CH_CTL_INTERRUPT |
diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
index 19ba7c7cbaab..de6d70a29b47 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
@@ -123,10 +123,18 @@ intel_dp_set_lttpr_transparent_mode(struct intel_dp *intel_dp, bool enable)
  */
 int intel_dp_lttpr_init(struct intel_dp *intel_dp)
 {
+	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 	int lttpr_count;
 	bool ret;
 	int i;
 
+	/*
+	 * Detecting LTTPRs must be avoided on platforms with an AUX timeout
+	 * period < 3.2ms. (see DP Standard v2.0, 2.11.2, 3.6.6.1).
+	 */
+	if (INTEL_GEN(i915) < 10)
+		return 0;
+
 	if (intel_dp_is_edp(intel_dp))
 		return 0;
 
-- 
2.25.1