mac mini (x86) hangs when booting w/out monitor

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

I have a Late 2014 mini (7,1) that runs NetBSD 10.

I can't seem to boot it headless.  As far as I can tell the kernel
hangs attaching video.  The machine hums and generates heat and never
completes the boot.  After power cycling and re-booting it (with a
monitor attached) the root fs is clean, i.e. the headless boot has
never got far enough to mount root.

One quirk I noticed about booting NetBSD on this machine is that that
headphones socket at the back glows red under NetBSD (i.e., I gather
it misdetects it as digital audio).  With the failed headless boot
when the kernel is stuck the audio jack starts glowing red, so the
kernel makes enough progress to probe it and that happens after
i915drmkms0.

I haven't yet tried booting a current kernel on it.  Tips, and
suggestions of things to test are welcome.  The full dmesg is at
nycbug, relevant excerpts are quoted below.  The machine dual-boots
Ubuntu, so a Linux dmesg is available for reference (linux boots
headless fine, the audio jack is not red under linux).


https://dmesgd.nycbug.org/dmesgd?do=view&id=8565

acpivga0 at acpi0 (IGPU): ACPI Display Adapter
acpiout0 at acpivga0 (DD01, 0x0100): ACPI Display Output Device
acpiout1 at acpivga0 (DD02, 0x0400): ACPI Display Output Device
acpiout2 at acpivga0 (DD03, 0x0200): ACPI Display Output Device
acpiout3 at acpivga0 (DD04, 0x0004): ACPI Display Output Device
acpiout4 at acpivga0 (DD05, 0x0005): ACPI Display Output Device
acpiout5 at acpivga0 (DD06, 0x0006): ACPI Display Output Device
acpiout6 at acpivga0 (DD07, 0x0007): ACPI Display Output Device
acpiout7 at acpivga0 (DD08, 0x0008): ACPI Display Output Device
acpiout8 at acpivga0 (DD1F, 0x001f): ACPI Display Output Device
acpivga0: connected output devices:
acpivga0:   0x0400 (acpiout1): Unknown Output Device, head 0
...
i915drmkms0 at pci0 dev 2 function 0: Intel Iris Graphics 5100 (rev. 0x09)
hdaudio0 at pci0 dev 3 function 0: HD Audio Controller
hdaudio0: interrupting at msi0 vec 0
hdaudio0: HDA ver. 1.0, OSS 2, ISS 0, BSS 0, SDO 1, 64-bit
hdafg0 at hdaudio0: Intel HDMI/DP
hdafg0: DP00 8ch: Digital Out [Jack]
hdafg0: 8ch/0ch 48000Hz PCM16*
audio0 at hdafg0: playback
audio0: slinear_le:16 2ch 48000Hz, blk 1920 bytes (10ms) for playback
...
hdaudio1 at pci0 dev 27 function 0: HD Audio Controller
hdaudio1: interrupting at msi2 vec 0
hdaudio1: HDA ver. 1.0, OSS 4, ISS 4, BSS 0, SDO 1, 64-bit
hdafg1 at hdaudio1: vendor 1013 product 4208
hdafg1: DAC00 2ch: Speaker [Built-In]
hdafg1: DAC01 2ch: HP Out [Jack]
hdafg1: DIG02 2ch: SPDIF Out [Jack]
hdafg1: ADC03 2ch: Mic In [Jack]
hdafg1: 2ch/2ch 48000Hz PCM16*
audio1 at hdafg1: playback, capture, full duplex, independent
audio1: slinear_le:16 2ch 48000Hz, blk 1920 bytes (10ms) for playback
audio1: slinear_le:16 2ch 48000Hz, blk 1920 bytes (10ms) for recording


-uwe