When I received the new Raspberry Pi 4 Model B (with 4GB RAM), I thought it would be as easy as taking the SD card from a Raspberry Pi 3 and put it in the new version 4. Well, close but not exact. You will have to copy over some additional files for the boot partition. I managed this by simply taking the actual available Raspbian image, extracting it to a new SD card, booting it up (for verification) and then copying over the complete boot partition.
This should be enough in general. Of course, in my case - with an unconventional LVM setup - it wasn’t. I also had to rebuild the initramfs boot image. That itself is fairly easy. Unfortunately - with some more files now on /boot - there wasn’t enough space anymore.
From here, there are multiple ways to go. For me, the task was clear. I added a new flag to bcrm, cloned the SD card from Pi 3 and increased the boot partition to 256 MB. That’s the recommended size (otherwise rpi-update` will issue a warning). Then I copied over the new files, rebuild the ‘initramfs’ file and now all is well.
New Kernel
When you have copied over all files, you will see that there is a new Kernel file available. On my Pi 3 installation, /boot had two files:
kernel7.img
kernel.img
Now, there is a third one:
kernel7l.img
Also, if you check a clean new installation, you’ll see that /lib/modules will have an additional folder, with an l suffix. For instance:
4.19.57+
4.19.57-v7+
4.19.57-v7l+
4.19.57-v7l+ is the new kernel version for Pi 4. Therefore, the ‘initramfs’ file has to be rebuilt, otherwise the boot process will fail because of inconsistent versions.
What I did - some more details
So, to summarize: I took three SD cards. On the first (/dev/mmcblk0) I simply installed the latest Raspbian image and booted my new Pi 4 from it. The second (/dev/sda) had the version 3 installation on it, and the third (/dev/sdb) was for the clone. After cloning, I mounted the boot partition (/dev/sdb1) and copied over all new files from /boot (/dev/mmcblk0p1). I then rebuilt ‘initramfs’:
sudo mkinitramfs -o /boot/initramfs.gz 4.19.57-v7l+
Afterwards I put the cloned card in the new Pi 4 and booted it up. All went fine. But, the kernel used from the plain installation image was a bit old. In addition, the current installation was still missing the -v7l+ module. For instance, my /lib/modules folder still had the following folders:
4.19.66+
4.19.66-v7+
Of course, I could have copied it over, but either way I prefer doing a clean upgrade. So, I checked https://github.com/Hexxeh/rpi-firmware/commits/master for the latest kernel. At the time of this writing, it was 4.19.69. The commit hash was f8c5a8734cde51ab94e07c204c97563a65a68636. So I executed:
sudo rpi-update f8c5a873
You do not have to provide the full hash, the first 6 or more digits will do just fine. And again, I rebuilt (like always) the initramfs file:
mkinitramfs -o /boot/initramfs.gz 4.19.69-v7l+
Another reboot, and I was ready to see what this new version 4 is capable of ;-)
Alternative approach
In my approach above I used 3 SD cards. I could have simply used one. In that case I would have created a backup of my original SD card first (instead of cloning). Then I could have mounted the Raspian image file and simply copied over the files from the boot partition. Maybe next time I’ll do exactly that one and expand this section here ;-)