On Thu, Jan 22, 2026 at 02:17:28PM +0100, Rasmus Villemoes wrote: > The am335x knows nothing about eMMC boot partitions, so in order to > implement an update procedure of the bootloader which is robust > against power failure or other interruptions, one must make use of the > fact that ROM code on the am335x looks for a valid first stage > bootloader at several different offsets. Updating that can then be > implemented by (assume we put MLO at offsets 128K and 256K): > > (1) At least one of the two slots must contain a valid header, since > we successfully booted. Pick the other one. > > (2) Overwrite the first sector of the slot chosen in step (a) with all > zeroes. > > (3) Write everything but the first sector to the chosen slot. > > (4) Write the first sector (containing the magic signature that the > boot ROM uses to identify a valid image) to the chosen slot. > > (5) Repeat steps (2)-(4) for the other slot. > > It's not possible to simply write the whole MLO in one go, especially > not when updating the 128K slot, because an interruption after the > first sector is written would make the ROM code believe the image is > valid. > > But this only caters for SPL itself; regardless of where SPL was > loaded from, it would go on to load U-Boot proper from > SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR. So in order to update the whole > bootloader, we need to teach SPL to use a > different offset for U-Boot proper depending on where SPL itself was > loaded from (*). With that, the update procedure is just amended by a step > > (3a) Write U-Boot proper to the offset corresponding to the SPL slot > being updated. > > We can know (*) because the ROM code sets a new bit in a certain > "trace vector" before each successive attempt. > > Signed-off-by: Rasmus Villemoes <[email protected]> > --- > arch/arm/mach-omap2/Kconfig | 37 +++++++++++++++++++++++++++++++ > arch/arm/mach-omap2/boot-common.c | 31 ++++++++++++++++++++++++++ > 2 files changed, 68 insertions(+)
Very interesting. I think the only thing really missing is that this needs to be covered under doc/ as well. -- Tom
signature.asc
Description: PGP signature
