Re: [PATCH] drm/i915/ddi: add DDI A force 4 lanes quirk for Apple MacBookPro11,5

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

On Sat, 12 Sep 2026, Theo Andersen Carton <andersen.theo@gmail.com> wrote:
> On dual-GPU Retina MacBooks the internal panel is muxed to the discrete
> GPU at boot, so Apple's firmware never lights up eDP on the Intel side.
> As a result it never sets DDI_A_4_LANES, and intel_ddi_max_lanes() reads
> back a source maximum of two lanes for port A.
>
> intel_dp_max_common_lane_count() then takes min3(source_max = 2,
> sink_max = 4, ...) = 2, giving 2 x 2.7 Gbps x 8/10 = 4.32 Gbps of link
> bandwidth. The panel's native 2880x1800@60 8bpc mode needs 8.1 Gbps, so
> every mode is rejected with MODE_CLOCK_HIGH and the connector comes up
> with no usable modes at all -- the panel stays dark whenever i915 is
> made to drive it, which looks like a hardware limitation but is not.
>
> The lanes are physically there. Reading the panel's DPCD from the AMD
> side, where amdgpu drives it correctly, reports MAX_LANE_COUNT = 0x84,
> i.e. 4 lanes, and amdgpu's own link_settings shows it running the panel
> at 4 lanes / HBR. macOS likewise drives this panel from the iGPU at its
> native resolution, so all four lanes are routed through the gmux to
> DDI A. Only the register bit is unset.
>
> intel_ddi_a_force_4_lanes() already exists for exactly this situation --
> its neighbouring comment notes that "Some BIOS might fail to set this
> bit on port A if eDP wasn't lit up at boot" -- but it only covers
> Broxton and Geminilake. Extend it with a DMI quirk so affected Macs get
> the correct lane count too.
>
> The DMI match is deliberately narrow. Other dual-GPU models in the same
> family are very likely affected identically, but they are untested, so
> widen the match only as reports come in.
>
> Tested on a MacBookPro11,5 (Broadwell, Iris Pro + Radeon R9 M370X):
> with the quirk, DDI A comes up as 4 lanes, link training passes at
> link rate 270000 / lane count 4, and i915 drives the internal panel at
> its native 2880x1800.
>
> Signed-off-by: Theo Andersen Carton <andersen.theo@gmail.com>

I'm guessing this is v2 of [1], but I shouldn't have to guess.

For future reference, please indicate patch revision, and log the
differences between the revisions. See Documentation/process, other
messages on the mailing list and git log for plenty of examples.

Is there a bug report about this? Would be nice to see the logs in the
failing case. See [2].


BR,
Jani.


[1] https://lore.kernel.org/r/CANFnrspHiK7ii28XPdLkOMyhSZcJ9xz6CT-kNX+oiiF+8N-2Fg@mail.gmail.com
[2] https://drm.pages.freedesktop.org/intel-docs/how-to-file-i915-bugs.html


> ---
>  drivers/gpu/drm/i915/display/intel_ddi.c    |  7 +++++
>  drivers/gpu/drm/i915/display/intel_quirks.c | 34 +++++++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_quirks.h |  1 +
>  3 files changed, 42 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c b/drivers/gpu/drm/i915/display/intel_ddi.c
> index 02a53c9848e1..f746287830b3 100644
> --- a/drivers/gpu/drm/i915/display/intel_ddi.c
> +++ b/drivers/gpu/drm/i915/display/intel_ddi.c
> @@ -4976,6 +4976,13 @@ static bool intel_ddi_a_force_4_lanes(struct intel_digital_port *dig_port)
>  	if (display->platform.geminilake || display->platform.broxton)
>  		return true;
>  
> +	/*
> +	 * Machines whose firmware leaves eDP dark on the Intel side never set
> +	 * the bit, so trust the quirk rather than the register.
> +	 */
> +	if (intel_has_quirk(display, QUIRK_DDI_A_FORCE_4_LANES))
> +		return true;
> +
>  	return false;
>  }
>  
> diff --git a/drivers/gpu/drm/i915/display/intel_quirks.c b/drivers/gpu/drm/i915/display/intel_quirks.c
> index 33245f44c0d5..feb28956a013 100644
> --- a/drivers/gpu/drm/i915/display/intel_quirks.c
> +++ b/drivers/gpu/drm/i915/display/intel_quirks.c
> @@ -100,6 +100,20 @@ static void quirk_disable_psr2(struct intel_display *display)
>  	drm_info(display->drm, "PSR2 support not currently available for this setup, applying disable PSR2 quirk\n");
>  }
>  
> +/*
> + * Dual-GPU Macs boot with the internal panel muxed to the discrete GPU, so
> + * the firmware never lights up eDP on the Intel side and consequently never
> + * sets DDI_A_4_LANES -- even though all four lanes are wired through the gmux
> + * to DDI A. Without the bit, port A is capped at two lanes, which is not
> + * enough bandwidth for the panel's native mode, leaving the connector with no
> + * usable modes at all.
> + */
> +static void quirk_ddi_a_force_4_lanes(struct intel_display *display)
> +{
> +	intel_set_quirk(display, QUIRK_DDI_A_FORCE_4_LANES);
> +	drm_info(display->drm, "Applying DDI A force 4 lanes quirk\n");
> +}
> +
>  struct intel_quirk {
>  	int device;
>  	int subsystem_vendor;
> @@ -142,6 +156,13 @@ static int intel_dmi_no_pps_backlight(const struct dmi_system_id *id)
>  	return 1;
>  }
>  
> +static int intel_dmi_ddi_a_force_4_lanes(const struct dmi_system_id *id)
> +{
> +	DRM_INFO("DDI A is 4 lanes despite firmware on %s\n", id->ident);
> +
> +	return 1;
> +}
> +
>  static const struct intel_dmi_quirk intel_dmi_quirks[] = {
>  	{
>  		.dmi_id_list = &(const struct dmi_system_id[]) {
> @@ -188,6 +209,19 @@ static const struct intel_dmi_quirk intel_dmi_quirks[] = {
>  		},
>  		.hook = quirk_no_pps_backlight_power_hook,
>  	},
> +	{
> +		.dmi_id_list = &(const struct dmi_system_id[]) {
> +			{
> +				.callback = intel_dmi_ddi_a_force_4_lanes,
> +				.ident = "Apple MacBookPro11,5",
> +				.matches = {DMI_EXACT_MATCH(DMI_SYS_VENDOR, "Apple Inc."),
> +					    DMI_EXACT_MATCH(DMI_PRODUCT_NAME, "MacBookPro11,5"),
> +				},
> +			},
> +			{ }
> +		},
> +		.hook = quirk_ddi_a_force_4_lanes,
> +	},
>  };
>  
>  static struct intel_quirk intel_quirks[] = {
> diff --git a/drivers/gpu/drm/i915/display/intel_quirks.h b/drivers/gpu/drm/i915/display/intel_quirks.h
> index 970a4fe52faf..d2b8e9183c62 100644
> --- a/drivers/gpu/drm/i915/display/intel_quirks.h
> +++ b/drivers/gpu/drm/i915/display/intel_quirks.h
> @@ -23,6 +23,7 @@ enum intel_quirk_id {
>  	QUIRK_EDP_LIMIT_RATE_HBR2,
>  	QUIRK_DISABLE_EDP_PANEL_REPLAY,
>  	QUIRK_DISABLE_PSR2,
> +	QUIRK_DDI_A_FORCE_4_LANES,
>  };
>  
>  void intel_init_quirks(struct intel_display *display);

-- 
Jani Nikula, Intel