RPi 3 kernel without VFAT support

Question

I am building a very compact Linux kernel for my RPi 3.

I have stripped a lot of features from the kernel. The current kernel size is around 4 MB.

Currently I only have ext4 and vfat FS enabled. I tried formatting /boot as ext4 to get rid of vfat code in kernel, but the system wouldn't boot.

After some research, I learnt that the RPi bootloader requires a vfat /boot partition. VFAT itself requires NLS features. => Large size

Any way I can get away with an ext4 /boot partition? I am building an embedded setup so the eventual plan is to get the RPi boot from a single Squash file system that would contain / and /boot in the same partition. So the final kernel would only have to support SquashFS.

Answer 1

You can pull the FAT partition out of your Linux system without a problem but you would need to do some stuff with a loader for Linux.

Okay the basic here is the ARM CPU(s) are co-processor to the GPU and the GPU needs a FAT partition to load the ARM code up before releasing them.

The GPU has its own FAT reader and it reads four mandatory files which must reside on a FAT partition

1. bootcode.bin
2. fixup.dat
3. start.elf
4. kernel.img

The kernel.img is our user code and you can basically do whatever you want from there.

So the bottom line here is there has to be a FAT on the card but no you don't have to be able to read FAT in your code. If you look at any bare metal code on the Pi I assure you that almost none have a FAT partition reader. The thing that reads the FAT for them is the GPU.

So long as Linux is happy to leave the FAT partition alone and you have a loader that can pass into linux, then Linux itself requires no understanding of the FAT system.

I suspect you will have to put the driver tree files on your Linux partition and load them at the startup of linux because I suspect Linux will need them.

If I was going to do it I would compile code for the linux loader for the Linux file system. Place it on the fat partition and write a small piece of bare metal code that simply places the file at 0x8000 onwards in the Pi and then jump the cpu0 to 0x8000 and it should run straight into linux without ever knowing about FAT. Any reasonable bare metal coder would be able to write that code and know what it does.

Answer 2

I looked into something similar to this briefly some time ago. My conclusion was that RPi hardware requires a vfat partition for booting, and this is an infrangible requirement, for all practical purposes anyway.

That said, you can use other bootloaders (e.g. U-Boot), which may or may not buy you something. The best source I've found on this subject is the RPi U-Boot page. I'd encourage to read through the documentation here; it's always possible that someone has "broken the code" since I last checked, and discovered a way around an infrangible rule :) Even if they haven't, some of the information here is useful; e.g. there is a (very) brief summary of the boot process.

You may also wish to consult some of the documentation The Organization publishes; e.g. Raspberry Pi Boot Modes describes all published boot modes, and contains some interesting options that you might find useful, and Boot Options for the config.txt file may also be useful. And finally, while I'm blethering here, I'll mention that there are options for network booting in addition to the USB and HDD options.

So, not the answer you were hoping for I fear, but hopefully this will give you something to ponder.

Answer 3

I doubt it. I think the SoC boot loader needs FAT. You have no control over the SoC boot loader.