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doc: update README.arm64 

This file apparently hasn't seen an update in a while, so just sync
it with reality.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
This commit is contained in:
Andre Przywara 2016-11-03 01:01:50 +00:00 committed by Tom Rini
parent 2a380cccc2
commit 8add67911d
1 changed files with 17 additions and 10 deletions
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doc/README.arm64
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@ -2,23 +2,31 @@ U-Boot for arm64
Summary
=======
No hardware platform of arm64 is available now. The U-Boot is
simulated on Foundation Model and Fast Model for ARMv8.
The initial arm64 U-Boot port was developed before hardware was available,
so the first supported platforms were the Foundation and Fast Model for ARMv8.
These days U-Boot runs on a variety of 64-bit capable ARM hardware, from
embedded development boards to servers.
Notes
=====
1. Currenly, U-Boot run at the highest exception level processor
supported and jump to EL2 or optionally EL1 before enter OS.
1. U-Boot can run at any exception level it is entered in, it is
recommened to enter it in EL3 if U-Boot takes some responsibilities of a
classical firmware (like initial hardware setup, CPU errata workarounds
or SMP bringup). U-Boot can be entered in EL2 when its main purpose is
that of a boot loader. It can drop to lower exception levels before
entering the OS.
2. U-Boot for arm64 is compiled with AArch64-gcc. AArch64-gcc
use rela relocation format, a tool(tools/relocate-rela) by Scott Wood
is used to encode the initial addend of rela to u-boot.bin. After running,
the U-Boot will be relocated to destination again.
3. Fdt should be placed at a 2-megabyte boundary and within the first 512
megabytes from the start of the kernel image. So, fdt_high should be
defined specially.
3. Earlier Linux kernel versions required the FDT to be placed at a
2 MB boundary and within the same 512 MB section as the kernel image,
resulting in fdt_high to be defined specially.
Since kernel version 4.2 Linux is more relaxed about the DT location, so it
can be placed anywhere in memory.
Please reference linux/Documentation/arm64/booting.txt for detail.
4. Spin-table is used to wake up secondary processors. One location
@ -37,9 +45,8 @@ Notes
aarch32 specific codes.
Contributor
===========
Contributors
============
Tom Rini <trini@ti.com>
Scott Wood <scottwood@freescale.com>
York Sun <yorksun@freescale.com>