Pi 400

Using a rom with the latest changes, the identical SD card image behaves differently on the Pi 400 than on the Pi 4. EEPROM version on both is the same (22 Oct) but the file ‘bcm-2711-rpi-4-b/dtb’, if present in the FAT partition prevents RISC OS from starting on the Pi 400 but is tolerated or ignored on the Pi 4. (The file concerned is present in the FAT partition on the RasPiOS distro and might therefore be downloaded if the user updated the firmware.)

After a lot of thinking and reading it appears that the default device tree (bcm2711-rpi-4-b/dtb) which is loaded (if present) and passed to the kernel (with any ramfs (a.k.a. CMOS), memory configuration and command line string data substituted) as an alternative to filling memory from 0×100 with ATAG data causes no difficulty on a Pi 4. Device Tree and ATAGs are mutually exclusive, and passing a DT blob to a kernel that doesn’t understand it will cause a boot failure. On a Pi 400 this is what happpens. The firmware will always try to load the DT (if it is present in the FAT partition) and pass it to the kernel.

To solve this, add the command ‘device_tree=’ to the conditional part of config.txt that applies to RISC OS.

To solve this, add the command ‘device_tree=’ to the conditional part of config.txt that applies to RISC OS.

That sounds interesting. How do I identify the conditional part of config.txt that applies to RISC OS?

Read the article in Archive 25:2 :)

What I did was to put the bit of CONFIG.TXT that applies to RISC OS in a condition like this:

[gpio5=1]
fake_vsync_isr=1
framebuffer_swap=0
gpu_mem=64
init_emmc_clock=100000000
ramfsfile=CMOS
ramfsaddr=0x508000
kernel=RISCOS.IMG
device_tree=
hdmi_drive=2
hdmi_blanking=1
disable_overscan=1
[pi4]
enable_gic=1
[all]
[gpio5=0]
# Additional overlays and parameters are documented /boot/overlays/README
# Enable audio (loads snd_bcm2835)
dtparam=audio=on
[pi4]
# Enable DRM VC4 V3D driver on top of the dispmanx display stack
dtoverlay=vc4-fkms-v3d
max_framebuffers=2
[all]
#dtoverlay=vc4-fkms-v3d

and then put the bit that starts RasPiOS in another conditional (which is only used if the switch between pins 29 and 30 on the header is closed)

The file CMDLINE.TXT has the magic incarnation that starts RasPiOS:

disable_gamma console=serial0,115200 console=tty1 root=PARTUUID=12345678-02
rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait quiet splash
plymouth.ignore-serial-consoles

and the RasPiOS distro is on a USB pen drive with the UUID given.

Wouldn’t it be easier to use an inexpensive USB3 hub with individual power switches for each port. Just a case of turning on the power to the device you want to boot from.

I wasn’t aiming for ‘easier’ I was aiming for ‘interesting and difficult’ with no trailing leads. In any case the final plan is to have three partitions on a single SD card – a solution that would work neatly on a Compute Module 4 with eMMC.

What I did was to put the bit of CONFIG.TXT that applies to RISC OS in a condition like this:

OK Thanks. Not quite what I thought you meant. But, I can see how that works.