From 9703d9d7f77ce129621f7d80a844822e2daa7008 Mon Sep 17 00:00:00 2001 From: Catalin Marinas Date: Mon, 5 Mar 2012 11:49:27 +0000 Subject: arm64: Kernel booting and initialisation The patch adds the kernel booting and the initial setup code. Documentation/arm64/booting.txt describes the booting protocol on the AArch64 Linux kernel. This is subject to change following the work on boot standardisation, ACPI. Signed-off-by: Will Deacon Signed-off-by: Catalin Marinas Acked-by: Nicolas Pitre Acked-by: Tony Lindgren Acked-by: Olof Johansson Acked-by: Santosh Shilimkar Acked-by: Arnd Bergmann --- Documentation/arm64/booting.txt | 152 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 152 insertions(+) create mode 100644 Documentation/arm64/booting.txt (limited to 'Documentation/arm64') diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt new file mode 100644 index 000000000000..9c4d388daddc --- /dev/null +++ b/Documentation/arm64/booting.txt @@ -0,0 +1,152 @@ + Booting AArch64 Linux + ===================== + +Author: Will Deacon +Date : 07 September 2012 + +This document is based on the ARM booting document by Russell King and +is relevant to all public releases of the AArch64 Linux kernel. + +The AArch64 exception model is made up of a number of exception levels +(EL0 - EL3), with EL0 and EL1 having a secure and a non-secure +counterpart. EL2 is the hypervisor level and exists only in non-secure +mode. EL3 is the highest priority level and exists only in secure mode. + +For the purposes of this document, we will use the term `boot loader' +simply to define all software that executes on the CPU(s) before control +is passed to the Linux kernel. This may include secure monitor and +hypervisor code, or it may just be a handful of instructions for +preparing a minimal boot environment. + +Essentially, the boot loader should provide (as a minimum) the +following: + +1. Setup and initialise the RAM +2. Setup the device tree +3. Decompress the kernel image +4. Call the kernel image + + +1. Setup and initialise RAM +--------------------------- + +Requirement: MANDATORY + +The boot loader is expected to find and initialise all RAM that the +kernel will use for volatile data storage in the system. It performs +this in a machine dependent manner. (It may use internal algorithms +to automatically locate and size all RAM, or it may use knowledge of +the RAM in the machine, or any other method the boot loader designer +sees fit.) + + +2. Setup the device tree +------------------------- + +Requirement: MANDATORY + +The device tree blob (dtb) must be no bigger than 2 megabytes in size +and placed at a 2-megabyte boundary within the first 512 megabytes from +the start of the kernel image. This is to allow the kernel to map the +blob using a single section mapping in the initial page tables. + + +3. Decompress the kernel image +------------------------------ + +Requirement: OPTIONAL + +The AArch64 kernel does not currently provide a decompressor and +therefore requires decompression (gzip etc.) to be performed by the boot +loader if a compressed Image target (e.g. Image.gz) is used. For +bootloaders that do not implement this requirement, the uncompressed +Image target is available instead. + + +4. Call the kernel image +------------------------ + +Requirement: MANDATORY + +The decompressed kernel image contains a 32-byte header as follows: + + u32 magic = 0x14000008; /* branch to stext, little-endian */ + u32 res0 = 0; /* reserved */ + u64 text_offset; /* Image load offset */ + u64 res1 = 0; /* reserved */ + u64 res2 = 0; /* reserved */ + +The image must be placed at the specified offset (currently 0x80000) +from the start of the system RAM and called there. The start of the +system RAM must be aligned to 2MB. + +Before jumping into the kernel, the following conditions must be met: + +- Quiesce all DMA capable devices so that memory does not get + corrupted by bogus network packets or disk data. This will save + you many hours of debug. + +- Primary CPU general-purpose register settings + x0 = physical address of device tree blob (dtb) in system RAM. + x1 = 0 (reserved for future use) + x2 = 0 (reserved for future use) + x3 = 0 (reserved for future use) + +- CPU mode + All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError, + IRQ and FIQ). + The CPU must be in either EL2 (RECOMMENDED in order to have access to + the virtualisation extensions) or non-secure EL1. + +- Caches, MMUs + The MMU must be off. + Instruction cache may be on or off. + Data cache must be off and invalidated. + External caches (if present) must be configured and disabled. + +- Architected timers + CNTFRQ must be programmed with the timer frequency. + If entering the kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) + set where available. + +- Coherency + All CPUs to be booted by the kernel must be part of the same coherency + domain on entry to the kernel. This may require IMPLEMENTATION DEFINED + initialisation to enable the receiving of maintenance operations on + each CPU. + +- System registers + All writable architected system registers at the exception level where + the kernel image will be entered must be initialised by software at a + higher exception level to prevent execution in an UNKNOWN state. + +The boot loader is expected to enter the kernel on each CPU in the +following manner: + +- The primary CPU must jump directly to the first instruction of the + kernel image. The device tree blob passed by this CPU must contain + for each CPU node: + + 1. An 'enable-method' property. Currently, the only supported value + for this field is the string "spin-table". + + 2. A 'cpu-release-addr' property identifying a 64-bit, + zero-initialised memory location. + + It is expected that the bootloader will generate these device tree + properties and insert them into the blob prior to kernel entry. + +- Any secondary CPUs must spin outside of the kernel in a reserved area + of memory (communicated to the kernel by a /memreserve/ region in the + device tree) polling their cpu-release-addr location, which must be + contained in the reserved region. A wfe instruction may be inserted + to reduce the overhead of the busy-loop and a sev will be issued by + the primary CPU. When a read of the location pointed to by the + cpu-release-addr returns a non-zero value, the CPU must jump directly + to this value. + +- Secondary CPU general-purpose register settings + x0 = 0 (reserved for future use) + x1 = 0 (reserved for future use) + x2 = 0 (reserved for future use) + x3 = 0 (reserved for future use) -- cgit v1.2.3-59-g8ed1b