[PATCH 1/1] drm/i915/dp: Refine TPS4-based HBR3 rejection and add quirk to limit eDP to HBR2

[Ankit Nautiyal]

Refine the logic introduced in commit 584cf613c24a ("drm/i915/dp: Reject
HBR3 when sink doesn't support TPS4") to allow HBR3 on eDP panels that
report DPCD revision 1.4, even if TPS4 is not supported. This aligns with
the DisplayPort specification, which does not mandate TPS4 support for eDP
with DPCD rev 1.4.

This change avoids regressions on panels that require HBR3 to operate at
their native resolution but do not advertise TPS4 support.

Additionally, some ICL/TGL platforms with combo PHY ports suffer from
signal integrity issues at HBR3. While certain systems include a
Parade PS8461 mux to mitigate this, its presence cannot be reliably
detected. Furthermore, broken or missing VBT entries make it unsafe to
rely on VBT for enforcing link rate limits.

To address the HBR3-related issues on such platforms and eDP panels,
introduce a device specific quirk to cap the eDP link rate to HBR2
(540000 kHz). This will override any higher advertised rates from
the sink or DPCD for specific devices.

Currently, the quirk is added for Dell XPS 13 7390 2-in-1 which is
reported in gitlab issue #5969 [1].

[1] https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/5969
[2] https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14517

Fixes: 584cf613c24a ("drm/i915/dp: Reject HBR3 when sink doesn't support TPS4")
Cc: Jani Nikula <jani.nikula at linux.intel.com>
Cc: Ville Syrj_l_ <ville.syrjala at linux.intel.com>
Cc: <stable at vger.kernel.org> # v6.15+
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/5969
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14517
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal at intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c     | 31 +++++++++++++++++++--
 drivers/gpu/drm/i915/display/intel_quirks.c |  9 ++++++
 drivers/gpu/drm/i915/display/intel_quirks.h |  1 +
 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index f48912f308df..362e376fca27 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -171,6 +171,15 @@ int intel_dp_link_symbol_clock(int rate)
 	return DIV_ROUND_CLOSEST(rate * 10, intel_dp_link_symbol_size(rate));
 }
 
+static bool intel_dp_reject_hbr3_due_to_tps4(struct intel_dp *intel_dp)
+{
+	/* TPS4 is not mandatory for eDP with DPCD rev 1.4 */
+	if (intel_dp_is_edp(intel_dp) && intel_dp->dpcd[DP_DPCD_REV] == 0x14)
+		return false;
+
+	return !drm_dp_tps4_supported(intel_dp->dpcd);
+}
+
 static int max_dprx_rate(struct intel_dp *intel_dp)
 {
 	struct intel_display *display = to_intel_display(intel_dp);
@@ -187,13 +196,22 @@ static int max_dprx_rate(struct intel_dp *intel_dp)
 	 * HBR3 without TPS4, and are unable to produce a stable
 	 * output. Reject HBR3 when TPS4 is not available.
 	 */
-	if (max_rate >= 810000 && !drm_dp_tps4_supported(intel_dp->dpcd)) {
+	if (max_rate >= 810000 && intel_dp_reject_hbr3_due_to_tps4(intel_dp)) {
 		drm_dbg_kms(display->drm,
 			    "[ENCODER:%d:%s] Rejecting HBR3 due to missing TPS4 support\n",
 			    encoder->base.base.id, encoder->base.name);
 		max_rate = 540000;
 	}
 
+	/*
+	 * Some platforms + eDP panels may not reliably support HBR3
+	 * due to signal integrity limitations, despite advertising it.
+	 * Cap the link rate to HBR2 to avoid unstable configurations for the
+	 * known machines.
+	 */
+	if (intel_dp_is_edp(intel_dp) && intel_has_quirk(display, QUIRK_EDP_LIMIT_RATE_HBR2))
+		max_rate = min(max_rate, 540000);
+
 	return max_rate;
 }
 
@@ -4304,13 +4322,22 @@ intel_edp_set_sink_rates(struct intel_dp *intel_dp)
 			 * HBR3 without TPS4, and are unable to produce a stable
 			 * output. Reject HBR3 when TPS4 is not available.
 			 */
-			if (rate >= 810000 && !drm_dp_tps4_supported(intel_dp->dpcd)) {
+			if (rate >= 810000 && intel_dp_reject_hbr3_due_to_tps4(intel_dp)) {
 				drm_dbg_kms(display->drm,
 					    "[ENCODER:%d:%s] Rejecting HBR3 due to missing TPS4 support\n",
 					    encoder->base.base.id, encoder->base.name);
 				break;
 			}
 
+			/*
+			 * Some platforms cannot reliably drive HBR3 rates due to PHY limitations,
+			 * even if the sink advertises support. Reject any sink rates above HBR2 on
+			 * the known machines for stable output.
+			 */
+			if (rate >= 810000 &&
+			    intel_has_quirk(display, QUIRK_EDP_LIMIT_RATE_HBR2))
+				break;
+
 			intel_dp->sink_rates[i] = rate;
 		}
 		intel_dp->num_sink_rates = i;
diff --git a/drivers/gpu/drm/i915/display/intel_quirks.c b/drivers/gpu/drm/i915/display/intel_quirks.c
index a32fae510ed2..d2e16b79d6be 100644
--- a/drivers/gpu/drm/i915/display/intel_quirks.c
+++ b/drivers/gpu/drm/i915/display/intel_quirks.c
@@ -80,6 +80,12 @@ static void quirk_fw_sync_len(struct intel_dp *intel_dp)
 	drm_info(display->drm, "Applying Fast Wake sync pulse count quirk\n");
 }
 
+static void quirk_edp_limit_rate_hbr2(struct intel_display *display)
+{
+	intel_set_quirk(display, QUIRK_EDP_LIMIT_RATE_HBR2);
+	drm_info(display->drm, "Applying eDP Limit rate to HBR2 quirk\n");
+}
+
 struct intel_quirk {
 	int device;
 	int subsystem_vendor;
@@ -231,6 +237,9 @@ static struct intel_quirk intel_quirks[] = {
 	{ 0x3184, 0x1019, 0xa94d, quirk_increase_ddi_disabled_time },
 	/* HP Notebook - 14-r206nv */
 	{ 0x0f31, 0x103c, 0x220f, quirk_invert_brightness },
+
+	/* Dell XPS 13 7390 2-in-1 */
+	{ 0x8a12, 0x1028, 0x08b0, quirk_edp_limit_rate_hbr2 },
 };
 
 static const struct intel_dpcd_quirk intel_dpcd_quirks[] = {
diff --git a/drivers/gpu/drm/i915/display/intel_quirks.h b/drivers/gpu/drm/i915/display/intel_quirks.h
index cafdebda7535..06da0e286c67 100644
--- a/drivers/gpu/drm/i915/display/intel_quirks.h
+++ b/drivers/gpu/drm/i915/display/intel_quirks.h
@@ -20,6 +20,7 @@ enum intel_quirk_id {
 	QUIRK_LVDS_SSC_DISABLE,
 	QUIRK_NO_PPS_BACKLIGHT_POWER_HOOK,
 	QUIRK_FW_SYNC_LEN,
+	QUIRK_EDP_LIMIT_RATE_HBR2,
 };
 
 void intel_init_quirks(struct intel_display *display);
-- 
2.45.2