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Currently, the documented kernel entry requirements are not explicit about whether the kernel should be entered in ARM or Thumb, leading to an ambiguitity about how to enter Thumb-2 kernels. As a result, the kernel is reliant on the zImage decompressor to enter the kernel proper in the correct instruction set state. This patch changes the boot entry protocol for head.S and Image to be the same as for zImage: in all cases, the kernel is now entered in ARM. Documentation/arm/Booting is updated to reflect this new policy. A different rule will be needed for Cortex-M class CPUs as and when support for those lands in mainline, since these CPUs don't support the ARM instruction set at all: a note is added to the effect that the kernel must be entered in Thumb on such systems. Signed-off-by: Dave Martin <dave.martin@linaro.org> Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
172 lines
6.0 KiB
Plaintext
172 lines
6.0 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. 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/kernel-parameters.txt.
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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
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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).
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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. The recommended placement is in the first 16KiB of RAM
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with the caveat that it may not be located at physical address 0 since
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the kernel interprets a value of 0 in r2 to mean neither a tagged list
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nor a dtb were passed.
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5. 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 (at any location) and
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called there. Note that the kernel uses 16K of RAM below the image
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to store page tables. The recommended placement is 32KiB into RAM.
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In either 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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The CPU must be in SVC mode. (A special exception exists for Angel)
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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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- 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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