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