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- Recommend that the kernel be placed under 128MiB, this is required if CONFIG_AUTO_ZRELADDR=y and good (or at least not bad) advice even if CONFIG_AUTO_ZRELADDR=n. - Recommend that a zImage kernel be placed above 32MiB, this avoids the need to relocate prior to decompression, which can speed up boot. - Add basic info on the requirements when loading a raw (non-zImage) kernel which are stricter than the zImage requirements. - Recommend that the DTB be placed after the 128MiB boundary, avoiding any potential conflict with the kernel decompressor, and within the lowmem region. In practice it could follow the kernel loaded after 32MiB, assuming the kernel decompesses to less than 32MiB, but the 128MiB recommendation is simple and unambiguous. - Add similar recommendation regarding initramfs. Signed-off-by: Ian Campbell <ian.campbell@citrix.com> Cc: Nicolas Pitre <nicolas.pitre@linaro.org> Cc: Will Deacon <will.deacon@arm.com> Cc: Dave Martin <dave.martin@linaro.org> Cc: Grant Likely <grant.likely@secretlab.ca> Cc: Roy Franz <roy.franz@linaro.org> Cc: linux-arm-kernel@lists.infradead.org Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
213 lines
7.6 KiB
Text
213 lines
7.6 KiB
Text
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/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, 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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