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

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

On Mon, 14 Sep 2026, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> 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].

i915_vbt from debugfs would also be interesting.

The patch itself looks sane.

BR,
Jani.


>
>
> 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