From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mark Rutland <mark.rutland@arm.com>
Date: Tue, 24 Mar 2015 10:31:00 +0000
Subject: [U-Boot] 64Bit device tree compilation
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Hi,

> Maybe a dumb question, why do we need to have a 64-bit U-Boot for
> arm64? I don't see we ever created 64-bit U-Boot for ppc64.

In ARMv8 it's not possible to change the register width at an exception
level (i.e. you can't change 64->32 or vice-versa), and lower exception
levels cannot be wider (so if your code at EL3 is 32-bit, you cannot run
64-bit code at EL3, EL2, EL1, or EL0).

Therefore you need a purely 64-bit path from EL3 to EL2 or EL1N in order
to boot a 64-bit kernel, so the bootloader needs to be 64-bit.

Mark.