[PATCH v2 1/5] drm/dp: Always use enhanced framing for DPCD r1.2+

[Dmytro Malovanyi]

DP 1.2 section 2.2.1.1 says "An upstream device interoperating with a
downstream device with DPCD Revision of 1.2 or higher must enable
Enhanced Framing Mode". So for r1.2+ the DPCD revision is enough, the
DP_ENHANCED_FRAME_CAP bit does not need to be checked.

Some sinks get this bit wrong. The Xiaomi Monitor A24i (P24FBA-RAGL)
sets it in the base receiver caps, but clears it in the extended
receiver caps at 0x2200, which are the ones we use:

  base:     12 0a c4 01 01 00 01 80 02 00 06 00 00 00 81
  extended: 14 0a 44 00 01 00 01 80 02 02 06 00 00 00 81

Through a USB4 dock (CalDigit Element 5) the link trains without
enhanced framing (0x202-0x204: 77 77 81), but the sink never syncs to
the stream (SINK_STATUS 0x205: 00) and shows "No signal". Forcing RBR
does not change it. Connected directly to the laptop the same sink
syncs without enhanced framing, at RBR, HBR and HBR2.

Make drm_dp_enhanced_frame_cap() return true for DPCD r1.2+. This
changes behaviour for every driver using the helper, but only for sinks
which already violate the spec by not setting the bit.

Tested on i915 (Arrow Lake-U), with the dock the link now uses enhanced
framing (0x101: 84) and the sink syncs (0x205: 03).

Fixes: a1d926527b79 ("drm/i915: implement EXTENDED_RECEIVER_CAPABILITY_FIELD_PRESENT")
Cc: stable@xxxxxxxxxxxxxxx
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/17719
Suggested-by: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
Signed-off-by: Dmytro Malovanyi <dmitrymalovanyy@xxxxxxxxx>
---
 include/drm/display/drm_dp_helper.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
index 5b05a7fbe..4f20f60db 100644
--- a/include/drm/display/drm_dp_helper.h
+++ b/include/drm/display/drm_dp_helper.h
@@ -160,6 +160,10 @@ drm_dp_max_lane_count(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
 static inline bool
 drm_dp_enhanced_frame_cap(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
 {
+	/* DP 1.2 2.2.1.1: enhanced framing is mandatory for DPCD r1.2+ */
+	if (dpcd[DP_DPCD_REV] >= 0x12)
+		return true;
+
 	return dpcd[DP_DPCD_REV] >= 0x11 &&
 		(dpcd[DP_MAX_LANE_COUNT] & DP_ENHANCED_FRAME_CAP);
 }
-- 
2.50.1 (Apple Git-155)