Re: [PATCH] drm/i915/dp: Configure protocol converter only for TMDS outputs

[Conor Svensson]

Hi Jani,

A gentle follow-up on this patch. Following your advice on the
original RFC, I investigated the root cause and posted the findings
and testing results to the GitLab issue:

https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/17180#note_3679658

The revised patch replaces the device-specific quirk with a change to
the protocol-converter configuration. Feedback on the approach and
whether this belongs in i915 or the DP helper would be appreciated.

Thanks for your time,

Conor


On Thu, 24 Sept 2026 at 22:19, Conor Svensson <conor10@xxxxxxxxx> wrote:
>
> intel_dp_configure_protocol_converter() currently writes the HDMI/DVI
> output-selection control for every DP 1.3+ branch. The write is only
> meaningful for a TMDS downstream output; a native DisplayPort output should
> not be switched to the HDMI/DVI protocol-converter mode.
>
> An Anker 565 branch (Synaptics OUI 90:cc:24) changes its downstream port
> descriptor from DisplayPort to DVI immediately after this write. i915 then
> derives a 165 MHz TMDS limit on the next hotplug probe and drops the
> monitor's native modes. Skipping the write for non-TMDS outputs prevents
> the transition.
>
> The existing dfp.min_tmds_clock classification is nonzero for HDMI/DVI
> outputs and zero for native DisplayPort outputs, so retain the write for
> the protocol-converter cases that need it. Colour-conversion controls are
> unchanged.
>
> Tested on a ThinkPad L14 Gen 4 with an Anker 565 A8388 hub and a Dell
> S3422DWG:
>
> - native 3440x1440@59.973 after DP recovery
> - three USB-C reconnects on the first USB-C port
> - one test on the second USB-C port
> - suspend/resume on the second port
> - HDMI through the hub at 3440x1440@59.973, with HDMI-specific modes
>   present
>
> On the unpatched control kernel, the live descriptor changed from
> 08 f0 01 1e 00 00 00 00 to 0a 42 00 00 00 00 00 00 across the 0x3050 write.
>
> Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/17180
>
> Root-cause findings and validation details were added to the linked issue.
>
> Assisted-by: LLM Codex GPT-6
>
> Signed-off-by: Conor Svensson <conor10@xxxxxxxxx>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index ffddf4b33..cdc3d7786 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -4422,14 +4422,17 @@ void intel_dp_configure_protocol_converter(struct intel_dp *intel_dp,
>         if (!drm_dp_is_branch(intel_dp->dpcd))
>                 return;
>
> -       tmp = intel_dp_has_hdmi_sink(intel_dp) ? DP_HDMI_DVI_OUTPUT_CONFIG : 0;
> +       /* HDMI/DVI output selection only applies to TMDS downstream ports. */
> +       if (intel_dp->dfp.min_tmds_clock) {
> +               tmp = intel_dp_has_hdmi_sink(intel_dp) ? DP_HDMI_DVI_OUTPUT_CONFIG : 0;
>
> -       ret = drm_dp_dpcd_write_byte(&intel_dp->aux,
> -                                    DP_PROTOCOL_CONVERTER_CONTROL_0, tmp);
> -       if (ret < 0)
> -               drm_dbg_kms(display->drm,
> -                           "Failed to %s protocol converter HDMI mode\n",
> -                           str_enable_disable(intel_dp_has_hdmi_sink(intel_dp)));
> +               ret = drm_dp_dpcd_write_byte(&intel_dp->aux,
> +                                            DP_PROTOCOL_CONVERTER_CONTROL_0, tmp);
> +               if (ret < 0)
> +                       drm_dbg_kms(display->drm,
> +                                   "Failed to %s protocol converter HDMI mode\n",
> +                                   str_enable_disable(intel_dp_has_hdmi_sink(intel_dp)));
> +       }
>
>         if (crtc_state->sink_format == INTEL_OUTPUT_FORMAT_YCBCR420) {
>                 switch (crtc_state->output_format) {
>
> base-commit: 703cd271db3a35088460ab3fec0a96bd48e7fe04
> --
> 2.55.0
>


--

Conor Svensson

Schedule a Meeting

+44 (0) 7497 376 365

LinkedIn  X (Twitter)