Re: How to boot from SD but run from USB?

Lawrence D'Oliveiro <ldo@nz.invalid> wrote:

On Mon, 27 Jan 2025 14:04:07 +0000, The Natural Philosopher wrote:
 

Yup Arm/broadcomm based Pis 'do it their way'
 
It goes back to the chips inception as a set top box embedded processpor

 
Also remember that GRUB depends on BIOS or UEFI, and the former is x86-
specific -- not sure about the latter.

UEFI isn't x86 specific - Arm uses it as well.

Basically, every vendor’s ARM chipset came up with its own way of booting.
In the absence of a BIOS-style interface for querying what hardware is
available, the Linux kernel is built with a “device tree” structure that
hard-codes this information for your specific chipset.

Yes, although to note that there are typically several stages of booting.
The UEFI / u-boot / etc is often the second or third stage of booting.  They
come after things like the ROM first-stage bootloader then the second stage
bootloader loadered from flash, or similar.  Sometimes boot is handled by a
CPU that isn't the main one, such as a 'management' CPU or in the Pi
1/2/3's case the GPU.

In the classic Pi 1, the process is:
1st stage: ROM bootloader (enough to read SD card)
2nd stage: bootcode.bin on SD card (firmware for GPU)
3rd stage: start.elf on SD card (firmware for GPU)
4th stage: Linux kernel from SD card (for CPU)

In Pi 3 and later the ROM bootloader also knows enough to read USB sticks as
well as SD cards.  The early stage(s) also move to EEPROM on Pi 4 to make
them easier to update.

There is now an equivalent spec standardized for the ARM world (adaptation
of UEFI??), but I understand this is only in use on servers with AArch64,
and the Raspberry Pi predates it anyway.

It's possible to run another bootloader like u-boot or UEFI as your fourth
stage, ie:

1st stage: ROM bootloader (GPU)
2nd stage: bootcode.bin on SD card (GPU)
3rd stage: start.elf on SD card (GPU)
4th stage: UEFI / u-boot on SD card (CPU)
5th stage: Linux kernel (CPU)

This is useful if you need more flexible booting than the regular boot
process provides - eg you want to fetch the kernel from an NFS share, which
is something not supported in the standard Pi firmware.

If the board vendor puts the firmware for stages 1-4 into EEPROM/flash, the
board now supports UEFI but the boot process didn't change, it just hid the
first four stages from you.

Theo