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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Oct 30, 2026 at 01:36:31PM +0200, Jani Nikula wrote:
> On Fri, 30 Oct 2026, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> > 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@vger.kernel.org
> >> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/17719
> >> Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
> >> Signed-off-by: Dmytro Malovanyi <dmitrymalovanyy@gmail.com>
> >> ---
> >>  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.
> 
> Commit message not enough?

On its face the check looks completely redundant, so without further
justification someone is likely to just remove it.

> 
> I think the wording in the specs suggest that with DPCD r1.2+ you should
> just enable enhanced framing regardless of the cap.
> 
> BR,
> Jani.
> 
> >
> >> +	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)
> 
> -- 
> Jani Nikula, Intel

-- 
Ville Syrjälä
Intel