Re: [PATCH 2/2] drm/i915/display: Disable display for iMac's

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

On Mon, 26 Jan 2026, Atharva Tiwari <atharvatiwarilinuxdev@gmail.com> wrote:
>> Rendering and display are quite separate in the hardware and in the
>> driver. Perhaps you do not mean "rendering" here? Perhaps it can be used
>> for rendering but not display?
>
> Yeah, it can be used for rendering but not display, sorry for the wording.
>
>> Okay, so perhaps there's no eDP connected. But what about the other
>> connectors on the iGPU? What about everything else in the display
>> hardware?
>
> You cant use the iGPU to drive the display on any hardware (including external displays).
>
>> If you can figure out that it's specifically link training that fails (a
>> dmesg would be useful to show this) there clearly is display hardware,
>> right?
>
> As said the iGPU cant drive the display on all monitors (even on macOS), but still heres the dmesg before this patch:
>
> [    5.095489] i915 0000:00:02.0: [drm] Found cometlake (device ID 9bc8) integrated display version 9.00 stepping N/A
> [    5.096061] i915 0000:00:02.0: [drm] VT-d active for gfx access
> [    5.096102] i915 0000:00:02.0: [drm] Using Transparent Hugepages
> [    5.099214] i915 0000:00:02.0: Invalid PCI ROM header signature: expecting 0xaa55, got 0xffff
> [    5.099217] i915 0000:00:02.0: [drm] Failed to find VBIOS tables (VBT)
> [    5.099324] i915 0000:00:02.0: vgaarb: VGA decodes changed: olddecodes=io+mem,decodes=none:owns=io+mem
> [    5.100960] i915 0000:00:02.0: [drm] Finished loading DMC firmware i915/kbl_dmc_ver1_04.bin (v1.4)
> [    5.832214] i915 0000:00:02.0: [drm] [ENCODER:105:DDI A/PHY A] failed to retrieve link info, disabling eDP
> [    5.835756] i915 0000:00:02.0: [drm] Registered 3 planes with drm panic
> [    6.428455] i915 0000:00:02.0: [drm] Cannot find any crtc or sizes
> [    6.722404] i915 0000:00:02.0: [drm] Cannot find any crtc or sizes
>
>> There's at least three levels where this could be handled, depending on
>> details:
>> 
>> - Display probe (the patch at hand). Assumes there's no display
>>   hardware, at all. The driver doesn't touch the hardware, which
>>   continues to consume power, it's not put in low power states. Not
>>   optimal if there actually is display hardware.
>> 
>> - Display disable. See intel_display_device_enabled(). The driver takes
>>   over the hardware, puts it to sleep, but prevents all connectors from
>>   being connected.

At a glance, this seems like the appropriate level.

What if you drop patch 2 and supply i915.disable_display=1 module
parameter? Or return false from intel_display_device_enabled().


BR,
Jani.

>> 
>> - eDP disable. Add a quirk somewhere to enforce eDP is disconnected, but
>>   other connectors can be used.
>
> The main reason I sent this patch is that after i915 trys to probe the display,
> the dGPU (amdgpu) is no longer able to detect the internal display,
> resulting in a black screen.
>
>> It would be quite useful to indicate the PCI ID of the device in
>> question.
>
> 00:02.0 VGA compatible controller [0300]: Intel Corporation CometLake-S GT2 [UHD Graphics 630] [8086:9bc8] (rev 03) (prog-if 00 [VGA controller])
> 	Subsystem: Apple Inc. CometLake-S GT2 [UHD Graphics 630] [106b:ffff]

-- 
Jani Nikula, Intel