219 lines
7.9 KiB
Plaintext
219 lines
7.9 KiB
Plaintext
Booting ARM Linux
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=================
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Author: Russell King
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Date : 18 May 2002
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The following documentation is relevant to 2.4.18-rmk6 and beyond.
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In order to boot ARM Linux, you require a boot loader, which is a small
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program that runs before the main kernel. The boot loader is expected
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to initialise various devices, and eventually call the Linux kernel,
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passing information to the kernel.
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Essentially, the boot loader should provide (as a minimum) the
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following:
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1. Setup and initialise the RAM.
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2. Initialise one serial port.
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3. Detect the machine type.
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4. Setup the kernel tagged list.
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5. Load initramfs.
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6. Call the kernel image.
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1. Setup and initialise RAM
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---------------------------
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Existing boot loaders: MANDATORY
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New boot loaders: MANDATORY
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The boot loader is expected to find and initialise all RAM that the
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kernel will use for volatile data storage in the system. It performs
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this in a machine dependent manner. (It may use internal algorithms
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to automatically locate and size all RAM, or it may use knowledge of
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the RAM in the machine, or any other method the boot loader designer
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sees fit.)
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2. Initialise one serial port
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-----------------------------
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Existing boot loaders: OPTIONAL, RECOMMENDED
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New boot loaders: OPTIONAL, RECOMMENDED
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The boot loader should initialise and enable one serial port on the
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target. This allows the kernel serial driver to automatically detect
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which serial port it should use for the kernel console (generally
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used for debugging purposes, or communication with the target.)
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As an alternative, the boot loader can pass the relevant 'console='
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option to the kernel via the tagged lists specifying the port, and
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serial format options as described in
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Documentation/admin-guide/kernel-parameters.rst.
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3. Detect the machine type
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--------------------------
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Existing boot loaders: OPTIONAL
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New boot loaders: MANDATORY except for DT-only platforms
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The boot loader should detect the machine type its running on by some
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method. Whether this is a hard coded value or some algorithm that
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looks at the connected hardware is beyond the scope of this document.
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The boot loader must ultimately be able to provide a MACH_TYPE_xxx
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value to the kernel. (see linux/arch/arm/tools/mach-types). This
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should be passed to the kernel in register r1.
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For DT-only platforms, the machine type will be determined by device
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tree. set the machine type to all ones (~0). This is not strictly
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necessary, but assures that it will not match any existing types.
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4. Setup boot data
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------------------
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Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED
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New boot loaders: MANDATORY
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The boot loader must provide either a tagged list or a dtb image for
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passing configuration data to the kernel. The physical address of the
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boot data is passed to the kernel in register r2.
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4a. Setup the kernel tagged list
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--------------------------------
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The boot loader must create and initialise the kernel tagged list.
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A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
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The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag
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has the size field set to '2' (0x00000002). The ATAG_NONE must set
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the size field to zero.
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Any number of tags can be placed in the list. It is undefined
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whether a repeated tag appends to the information carried by the
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previous tag, or whether it replaces the information in its
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entirety; some tags behave as the former, others the latter.
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The boot loader must pass at a minimum the size and location of
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the system memory, and root filesystem location. Therefore, the
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minimum tagged list should look:
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+-----------+
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base -> | ATAG_CORE | |
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+-----------+ |
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| ATAG_MEM | | increasing address
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+-----------+ |
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| ATAG_NONE | |
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+-----------+ v
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The tagged list should be stored in system RAM.
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The tagged list must be placed in a region of memory where neither
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the kernel decompressor nor initrd 'bootp' program will overwrite
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it. The recommended placement is in the first 16KiB of RAM.
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4b. Setup the device tree
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-------------------------
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The boot loader must load a device tree image (dtb) into system ram
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at a 64bit aligned address and initialize it with the boot data. The
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dtb format is documented in Documentation/devicetree/booting-without-of.txt.
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The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
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physical address to determine if a dtb has been passed instead of a
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tagged list.
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The boot loader must pass at a minimum the size and location of the
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system memory, and the root filesystem location. The dtb must be
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placed in a region of memory where the kernel decompressor will not
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overwrite it, whilst remaining within the region which will be covered
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by the kernel's low-memory mapping.
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A safe location is just above the 128MiB boundary from start of RAM.
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5. Load initramfs.
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------------------
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Existing boot loaders: OPTIONAL
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New boot loaders: OPTIONAL
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If an initramfs is in use then, as with the dtb, it must be placed in
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a region of memory where the kernel decompressor will not overwrite it
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while also with the region which will be covered by the kernel's
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low-memory mapping.
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A safe location is just above the device tree blob which itself will
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be loaded just above the 128MiB boundary from the start of RAM as
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recommended above.
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6. Calling the kernel image
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---------------------------
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Existing boot loaders: MANDATORY
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New boot loaders: MANDATORY
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There are two options for calling the kernel zImage. If the zImage
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is stored in flash, and is linked correctly to be run from flash,
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then it is legal for the boot loader to call the zImage in flash
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directly.
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The zImage may also be placed in system RAM and called there. The
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kernel should be placed in the first 128MiB of RAM. It is recommended
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that it is loaded above 32MiB in order to avoid the need to relocate
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prior to decompression, which will make the boot process slightly
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faster.
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When booting a raw (non-zImage) kernel the constraints are tighter.
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In this case the kernel must be loaded at an offset into system equal
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to TEXT_OFFSET - PAGE_OFFSET.
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In any case, the following conditions must be met:
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- Quiesce all DMA capable devices so that memory does not get
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corrupted by bogus network packets or disk data. This will save
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you many hours of debug.
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- CPU register settings
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r0 = 0,
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r1 = machine type number discovered in (3) above.
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r2 = physical address of tagged list in system RAM, or
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physical address of device tree block (dtb) in system RAM
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- CPU mode
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All forms of interrupts must be disabled (IRQs and FIQs)
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For CPUs which do not include the ARM virtualization extensions, the
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CPU must be in SVC mode. (A special exception exists for Angel)
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CPUs which include support for the virtualization extensions can be
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entered in HYP mode in order to enable the kernel to make full use of
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these extensions. This is the recommended boot method for such CPUs,
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unless the virtualisations are already in use by a pre-installed
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hypervisor.
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If the kernel is not entered in HYP mode for any reason, it must be
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entered in SVC mode.
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- Caches, MMUs
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The MMU must be off.
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Instruction cache may be on or off.
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Data cache must be off.
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If the kernel is entered in HYP mode, the above requirements apply to
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the HYP mode configuration in addition to the ordinary PL1 (privileged
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kernel modes) configuration. In addition, all traps into the
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hypervisor must be disabled, and PL1 access must be granted for all
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peripherals and CPU resources for which this is architecturally
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possible. Except for entering in HYP mode, the system configuration
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should be such that a kernel which does not include support for the
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virtualization extensions can boot correctly without extra help.
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- The boot loader is expected to call the kernel image by jumping
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directly to the first instruction of the kernel image.
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On CPUs supporting the ARM instruction set, the entry must be
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made in ARM state, even for a Thumb-2 kernel.
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On CPUs supporting only the Thumb instruction set such as
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Cortex-M class CPUs, the entry must be made in Thumb state.
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