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diff --git a/kernel/Documentation/arm64/booting.txt b/kernel/Documentation/arm64/booting.txt new file mode 100644 index 000000000..f3c05b5f9 --- /dev/null +++ b/kernel/Documentation/arm64/booting.txt @@ -0,0 +1,224 @@ + Booting AArch64 Linux + ===================== + +Author: Will Deacon <will.deacon@arm.com> +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 placed on an 8-byte boundary within +the first 512 megabytes from the start of the kernel image and must not +cross a 2-megabyte boundary. 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 64-byte header as follows: + + u32 code0; /* Executable code */ + u32 code1; /* Executable code */ + u64 text_offset; /* Image load offset, little endian */ + u64 image_size; /* Effective Image size, little endian */ + u64 flags; /* kernel flags, little endian */ + u64 res2 = 0; /* reserved */ + u64 res3 = 0; /* reserved */ + u64 res4 = 0; /* reserved */ + u32 magic = 0x644d5241; /* Magic number, little endian, "ARM\x64" */ + u32 res5; /* reserved (used for PE COFF offset) */ + + +Header notes: + +- As of v3.17, all fields are little endian unless stated otherwise. + +- code0/code1 are responsible for branching to stext. + +- when booting through EFI, code0/code1 are initially skipped. + res5 is an offset to the PE header and the PE header has the EFI + entry point (efi_stub_entry). When the stub has done its work, it + jumps to code0 to resume the normal boot process. + +- Prior to v3.17, the endianness of text_offset was not specified. In + these cases image_size is zero and text_offset is 0x80000 in the + endianness of the kernel. Where image_size is non-zero image_size is + little-endian and must be respected. Where image_size is zero, + text_offset can be assumed to be 0x80000. + +- The flags field (introduced in v3.17) is a little-endian 64-bit field + composed as follows: + Bit 0: Kernel endianness. 1 if BE, 0 if LE. + Bits 1-63: Reserved. + +- When image_size is zero, a bootloader should attempt to keep as much + memory as possible free for use by the kernel immediately after the + end of the kernel image. The amount of space required will vary + depending on selected features, and is effectively unbound. + +The Image must be placed text_offset bytes from a 2MB aligned base +address near the start of usable system RAM and called there. Memory +below that base address is currently unusable by Linux, and therefore it +is strongly recommended that this location is the start of system RAM. +At least image_size bytes from the start of the image must be free for +use by the kernel. + +Any memory described to the kernel (even that below the 2MB aligned base +address) which is not marked as reserved from the kernel e.g. with a +memreserve region in the device tree) will be considered as available to +the kernel. + +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. + The address range corresponding to the loaded kernel image must be + cleaned to the PoC. In the presence of a system cache or other + coherent masters with caches enabled, this will typically require + cache maintenance by VA rather than set/way operations. + System caches which respect the architected cache maintenance by VA + operations must be configured and may be enabled. + System caches which do not respect architected cache maintenance by VA + operations (not recommended) must be configured and disabled. + +- Architected timers + CNTFRQ must be programmed with the timer frequency and CNTVOFF must + be programmed with a consistent value on all CPUs. 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. + + For systems with a GICv3 interrupt controller: + - If EL3 is present: + ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1. + ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b1. + - If the kernel is entered at EL1: + ICC.SRE_EL2.Enable (bit 3) must be initialised to 0b1 + ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b1. + +The requirements described above for CPU mode, caches, MMUs, architected +timers, coherency and system registers apply to all CPUs. All CPUs must +enter the kernel in the same exception level. + +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 + an 'enable-method' property for each cpu node. The supported + enable-methods are described below. + + It is expected that the bootloader will generate these device tree + properties and insert them into the blob prior to kernel entry. + +- CPUs with a "spin-table" enable-method must have a 'cpu-release-addr' + property in their cpu node. This property identifies a + naturally-aligned 64-bit zero-initalised memory location. + + These CPUs should 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 to this + value. The value will be written as a single 64-bit little-endian + value, so CPUs must convert the read value to their native endianness + before jumping to it. + +- CPUs with a "psci" enable method should remain outside of + the kernel (i.e. outside of the regions of memory described to the + kernel in the memory node, or in a reserved area of memory described + to the kernel by a /memreserve/ region in the device tree). The + kernel will issue CPU_ON calls as described in ARM document number ARM + DEN 0022A ("Power State Coordination Interface System Software on ARM + processors") to bring CPUs into the kernel. + + The device tree should contain a 'psci' node, as described in + Documentation/devicetree/bindings/arm/psci.txt. + +- 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) |