Nixpkgs: raspberry pi doesn't boot without monitor attached

[lheckemann]

Describe the bug
The 19.09 and unstable aarch64 sd images won't boot on the raspberry pi 3A+ (checked myself) and 3B+ (I think I heard this) if no monitor is connected to its HDMI port.

To Reproduce
Steps to reproduce the behavior:

1. Write 19.09 or unstable sd image to SD card
2. Insert SD card in raspberry pi
3. Power up with no monitor attached

Expected behavior
Boots.

Actual behaviour
Boot loop, where u-boot repeatedly resets when trying to initialise the graphical console.

Additional context
This works if the firmware in the pi boot partition is replaced with raspberrypifw from 19.03.

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: raspberrypifw
# a list of nixos modules affected by the problem
module:

cc @mmahut who I believe had this issue too.

[alexbakker]

I'm facing the same issue. This can also be worked around by setting hdmi_force_hotplug=1 in config.txt.

[kwohlfahrt]

Can confirm this on the 3B (not plus), and that the workaround above fixes it. I'll see if I can debug more, is there any upstream report?

For my notes: The earliest nixpkgs version I can confirm this was 30127503a5edd2da66f889e39e2c9b000d11a1bf, and 19.03 was still OK.

[hexop]

I have a 3B (not plus), and I tried the workaround adding:

~
boot.loader.raspberryPi.firmwareConfig = ''
hdmi_force_hotplug=1
'';
~

But it still doesn't boot if there's no HDMI plugged in. I have to add that I'm an absolute newbie to NixOS and I just set up the machine.

[craigem]

What happens if you write /boot/config.txt manually, @hexop ?

I've found that firmwareConfig appears to be broken in 19.09 (see #67792 ).

[hexop]

What happens if you write /boot/config.txt manually, @hexop ?

I've found that firmwareConfig appears to be broken in 19.09 (see #67792 ).

Well I've tried creating /boot/config.txt manually, and it still doesn't work. I've also tried removing the boot.loader.raspberryPi.firmwareConfig from configuration.nix but it still doesn't work.
Right now I have plugged an HDMI-to-VGA converter in the HDMI port and it powers on, without the need for a real monitor.

[alexbakker]

Try mounting mmcblk0p1 and changing the config.txt that's there, instead of the one on mmcblk0p2.

[hexop]

Try mounting mmcblk0p1 and changing the config.txt that's there, instead of the one on mmcblk0p2.

That worked perfectly! Thank you so much.

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