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

[Ville Syrjälä]

On Thu, Oct 29, 2026 at 11:18:22AM +0100, Dmytro Malovanyi wrote:
> 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+ */

DP_ENHANCED_FRAME_CAP is also mandatory so I think this needs
a bit more detailed explanation.

> +	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)

-- 
Ville Syrjälä
Intel