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U-Boot for arm64
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Summary
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=======
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The initial arm64 U-Boot port was developed before hardware was available,
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so the first supported platforms were the Foundation and Fast Model for ARMv8.
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These days U-Boot runs on a variety of 64-bit capable ARM hardware, from
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embedded development boards to servers.
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Notes
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=====
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1. U-Boot can run at any exception level it is entered in, it is
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recommened to enter it in EL3 if U-Boot takes some responsibilities of a
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classical firmware (like initial hardware setup, CPU errata workarounds
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or SMP bringup). U-Boot can be entered in EL2 when its main purpose is
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that of a boot loader. It can drop to lower exception levels before
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entering the OS.
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2. U-Boot for arm64 is compiled with AArch64-gcc. AArch64-gcc
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use rela relocation format, a tool(tools/relocate-rela) by Scott Wood
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is used to encode the initial addend of rela to u-boot.bin. After running,
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the U-Boot will be relocated to destination again.
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3. Earlier Linux kernel versions required the FDT to be placed at a
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2 MB boundary and within the same 512 MB section as the kernel image,
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resulting in fdt_high to be defined specially.
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Since kernel version 4.2 Linux is more relaxed about the DT location, so it
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can be placed anywhere in memory.
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Please reference linux/Documentation/arm64/booting.txt for detail.
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4. Spin-table is used to wake up secondary processors. One location
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(or per processor location) is defined to hold the kernel entry point
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for secondary processors. It must be ensured that the location is
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accessible and zero immediately after secondary processor
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enter slave_cpu branch execution in start.S. The location address
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is encoded in cpu node of DTS. Linux kernel store the entry point
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of secondary processors to it and send event to wakeup secondary
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processors.
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Please reference linux/Documentation/arm64/booting.txt for detail.
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5. Generic board is supported.
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6. CONFIG_ARM64 instead of CONFIG_ARMV8 is used to distinguish aarch64 and
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aarch32 specific codes.
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Contributors
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============
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Tom Rini <trini@ti.com>
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Scott Wood <scottwood@freescale.com>
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York Sun <yorksun@freescale.com>
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Simon Glass <sjg@chromium.org>
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Sharma Bhupesh <bhupesh.sharma@freescale.com>
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Rob Herring <robherring2@gmail.com>
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Sergey Temerkhanov <s.temerkhanov@gmail.com>
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