Mac mini 12+ DCP initialization fails with some monitors

I have two monitors: An iiyama ProLite E2473HDS (1080p) and a Philips 279P1 (4K). They do not work with m1n1 from a 12.1 Mac mini (connected over HDMI). When m1n1 starts, the 1080p monitor wakes up from sleep (power LED turns blue), and a few seconds later displays “No signal” and goes back to sleep. m1n1 logs (including the DCP syslog messages) indicate nothing out of the ordinary; the display is correctly recognized at the correct resolution/refresh rate. In fact, the log is identical to the one produced when the mini is connected to a 1080p capture card. The 4K monitor does not seem to even wake up.

I’ve messed with m1n1’s display init code (mostly with the 1080p monitor since it’s not my primary display) and found out the following:

• The monitor wakes up from sleep when dcp_ib_swap_begin is called.
• Calling dcp_ib_set_mode after dcp_ib_swap_begin does not change anything at that point.
• Calling dcp_ib_get_hpd in a loop after dcp_ib_swap_begin shows that connected goes back to 0 after 1-2 seconds, then goes back to 1 after ~400ms.
• Calling dcp_ib_set_mode after connected is back to 1, then completing the swap (dcp_ib_swap_set_layer/dcp_ib_swap_end) makes the monitor work correctly; it will actually show m1n1 logo/console. It still takes a couple of seconds after completing the swap for the image to actually appear.
• When rebooting, the situation is messier; after the call to dcp_ib_swap_begin, the monitor will still disconnect/reconnect, but after 5-6 seconds, not 1-2 seconds. It is, however, not necessary to wait for the disconnection: calling dcp_ib_set_mode a couple of seconds after dcp_ib_swap_begin seems to prevent the disconnection, and the monitor works afterwards.

I managed to make both my monitors work by making these changes: wait up to 5 seconds for the display to disconnect; if it does, wait for it to reconnect and call dcp_ib_set_mode; if it doesn’t, call dcp_ib_set_mode anyway.

This has the obvious issue of slowing down startup by at least 5 seconds on all monitors; it also seems very fragile and timing-dependent. I’m guessing there must be a better solution, since the monitors work (most of the time) in m1n1 with 11.6 iBoot, and it doesn’t seem to wait 5 seconds for the monitor to disconnect/reconnect… (I’ve tried to measure the time iBoot takes before starting m1n1, and it seems 11.6 is only slower than 12.1 by 900ms)

[alyssarosenzweig]

Once Linux has a real display driver it will work like macOS.

coughs it works for me

[marcan]

None of these displays “work” under macOS because Apple disabled iBoot display output entirely since 12.0 😉

There is no display output until the OS driver loads, which means no verbose boot, no iBoot messages, no Apple logo and no progress bar until the kernel is half booted, etc. In fact, you get those features back if you chainload macOS under m1n1 and the display was initialized properly.

Once Linux has a real display driver it will work like macOS. The problem we’re trying to solve in this bug is bootloader display output, which is handicapped by the iBoot-mode interface that Apple provides for it. That’s why we’re having so much trouble with modes and timeouts and such.