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

[Jani Nikula <jani.nikula@linux.intel.com>]

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?

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