How to boot RPi armv8 as aarch64 or armv7h?

[keithspg]

Can you have an armv6 or v7 chroot under aarch64? If so how? I tried to boot my armv6 chroot under aarch64 and it fails to launch. Is there some kernel option that can be set (requiring a kernel rebuild)? I ask this as I have a distccd build setup for Rpi1/2/4 and have recently added aarch64 to the mix. I'd like to have only one booted image (aarch64) and switch into chroots to build the packages for armv7 and armv6... Can I have a 'dual boot situation where I can boot it with a v7 kernel or a v8? Basically saving me from powering down and removing the card would be helpful as well.

Currently, I have an armv7 image and chroot into armv6 to build the armv6 packages and that works great as I can build armv6 almost as fast as armv7!

[graysky]

I do not believe you can. For building packages for armv6h, armv7h and aarch64 on my RPi4, I have 2 uSD cards, one running armv7h and another running aarch64. Maybe someone can correct me if there is a way to do it all from aarch64.

[keithspg]

So, I was thinking... I have an RPiB3+ running aarch64. It boots with uboot, right? I know very little about it, but I was wondering if I could create a mmcblkp3 as an archv7h filesystem and switch the kernel at boot time.

What I am thinking is: I boot as aarch64, it would boot from /boot with kernel8.img but mount its root from mmcblk0p2. As amv7h, its root would be on mmcblk0p3. I would select (rename?) an initramfs and kernel for the next boot then reboot. So it reboots as armv7h. How do I tell the boot code to use kernel7 versus kernel8?

Or maybe I am no longer using uboot. How do I figure that out?
My system is a bit different from standard arch arm aarch64: I downloaded the arch_latest_aarch64 image booted then built and installed a raspberrypi kernel that I compiled using moonman's PKGBUILD. I am running the same kernel revision 5.4.45-1-ARCH on armv7 and aarch64. I compare the files in /boot and they look to be all the same filesize and name. The only difference is kernel8.img versus kernel7.img.

aarch64: 5.4.45-1-ARCH #1 SMP PREEMPT Wed Jul 1 20:34:37 CDT 2020 aarch64 GNU/Linux
armv7h: 5.4.45-1-ARCH #1 SMP PREEMPT Thu Jul 2 10:25:37 CDT 2020 armv7l GNU/Linux

Can I do this with config.txt/cmdline.txt or is it uboot? I am lost...

Am I way off base here?

[keithspg]

Ok, I have found something of a 'work around' for my PiB3+.
I made an aarch64 image with my custom 5.4.45 kernel and booted it. I made a 3rd partition and copied my full armv7 installation to it. I copied my kernel7.img and initramfs-linux.img to /boot/armv7

I edited cmdline.txt and copied the line and commented out the first line and changed root to /dev/mmcblk0p3
$this->bbcode_second_pass_code('', '#root=/dev/mmcblk0p2 rw rootwait ...
root=/dev/mmcblk0p3 rw rootwait ...')

I edited config.txt and put this at the end:
$this->bbcode_second_pass_code('', '# armv8
#arm_64bit=1
#kernel=kernel8.img
# armv7
arm_64bit=0
kernel=kernel7.img

initramfs initramfs-linux.img followkernel')

To boot as armv7, I move kernel8.img and initramfs-linux.img to a folder /armv8 and put kernel7.img and its initramfs-linux.img into the /boot and uncomment the second line of the cmdline.txt and comment out the appropriate 2 lines of config.txt, voila, it boots as armv7 and I can access my chroot for armv6. To boot it back as aarch64, I remove the armv7 bits and comment out the second line of the cmdline.txt and swap back the comments in config.txt.

Is there an easier way? I will change the title of the thread as it has diverged from my original request

[keithspg]

I made it a bit easier by keeping the kernels in their respective folders. That way I do not need to cp/rm etc.:

$this->bbcode_second_pass_code('', '# armv8
arm_64bit=1
kernel=armv8/kernel8.img
initramfs armv8/initramfs-linux.img followkernel

# armv7
#arm_64bit=0
#kernel=armv7/kernel7.img
#initramfs armv7/initramfs-linux.img followkernel')

This way, I comment/uncomment in config.txt and in cmdline.txt. I have verified this works. I will make a script to make these changes to 2 txt files and it allows me to use one 32Gb card to compile for all 3 architectures!

Now if I could just figure out why aarch64 does not mount the nfs mount in the fstab at boot like armv7 does...

[graysky]

If you share the same /boot you will run into issues I think. There are files that get installed to /boot/overlays for example that might have the same name.

[keithspg]

I have already run into that and have mitigated it somewhat. You probably know better that I do, but the files appear to be the same size. I assume they are the same.
So far, I have been renaming any files pacman has problems with when needed and move the kernel.img and initramfs back to /boot before I update the kernel and back to their respective folders afterward. I have been keeping the same kernel revision for both architectures. Is there a better way? It would be nice to have a dual boot maybe even with nfs boot or uboot (though I do not really understand how that is implicated. When I remove the uboot package from the armv8 root, it fails to boot even for the armv7. I am really unsure of what kind of frankerstrin I have created, but it does allow me to do what I want, at least for now. The key is I need to be able to do it via ssh as it is headless.

Any suggestions?

Keith

[graysky]

Not sure what the best way to do what you want is... I just have 2 uSD card, one with armv7h and another with aarch64. It's pretty easy to swap them.