MidasBootloaderIncompatibleWithLinux¶

Introduction¶

The bootloader of the following devices is incompatible with upstream Linux:

Galaxy SIII (GT-I9300)
Galaxy SIII 4G (GT-I9305)
Galaxy Note II (GT-N7100)
Galaxy Note II 4G (GT-N7105)

When jumping to Linux, the booloader still has:

the instruction cache on
the data cache on
the MMU on

Documentation/arm/booting.rst which is there since 2003 states that:

"The MMU must be off."
"Data cache must be off."

And upstream Linux won't accept patches to disable the MMU, or the data cache in Linux as they have very good reasons to do that, and they are supposed to be already disabled.

Supporting s-boot¶

To workaround that:

However as the kernel evolved we need to patch it more and more:

The patch disabling the caches "was not necessary on Linux 3.4":lkml.iu.edu/hypermail/linux/kernel/1212.1/02099.html
Disabling CONFIG_GCC_PLUGINS was also not necessary on older kernel where the option was not even present

So it's not a viable option in the long run.

Longer term options¶

As u-boot disables the instruction cache in the boot commands before jumping to Linux, we might be able to replace the boot.img by u-boot to workaround the stock bootloader issues.

Having u-boot would also enable many benefits, such as the ability to have Replicant images that run on multiple devices.

As u-boot is capable of running in different ways, when upstreamming the code we will need to make sure that it can run as boot.img but also with the nonfree and non-redistributable BL1 because other people are interested in that and that, if we manage to get rid of the BL1 at some point, many of the upstreaming work will already been done.