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 thekernel will use for volatile data storage in the system. It performsthis in a machine dependent manner. (It may use internal algorithmsto automatically locate and size all RAM, or it may use knowledge ofthe RAM in the machine, or any other method the boot loader designersees 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 thetarget. This allows the kernel serial driver to automatically detectwhich serial port it should use for the kernel console (generallyused 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, andserial format options as described in

The kernel’s command-line parameters.

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 somemethod. Whether this is a hard coded value or some algorithm thatlooks at the connected hardware is beyond the scope of this document.The boot loader must ultimately be able to provide a MACH_TYPE_xxxvalue to the kernel. (see linux/arch/arm/tools/mach-types). Thisshould be passed to the kernel in register r1.

For DT-only platforms, the machine type will be determined by devicetree. set the machine type to all ones (~0). This is not strictlynecessary, 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 forpassing configuration data to the kernel. The physical address of theboot 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 taghas the size field set to ‘2’ (0x00000002). The ATAG_NONE must setthe size field to zero.

Any number of tags can be placed in the list. It is undefinedwhether a repeated tag appends to the information carried by theprevious tag, or whether it replaces the information in itsentirety; some tags behave as the former, others the latter.

The boot loader must pass at a minimum the size and location ofthe system memory, and root filesystem location. Therefore, theminimum 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 neitherthe kernel decompressor nor initrd ‘bootp’ program will overwriteit. 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 ramat a 64bit aligned address and initialize it with the boot data. Thedtb format is documented athttps://www.devicetree.org/specifications/.The kernel will look for the dtb magic value of 0xd00dfeed at the dtbphysical address to determine if a dtb has been passed instead of atagged list.

The boot loader must pass at a minimum the size and location of thesystem memory, and the root filesystem location. The dtb must beplaced in a region of memory where the kernel decompressor will notoverwrite it, while remaining within the region which will be coveredby 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 ina region of memory where the kernel decompressor will not overwrite itwhile also with the region which will be covered by the kernel’slow-memory mapping.

A safe location is just above the device tree blob which itself willbe loaded just above the 128MiB boundary from the start of RAM asrecommended 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 zImageis 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 flashdirectly.

The zImage may also be placed in system RAM and called there. Thekernel should be placed in the first 128MiB of RAM. It is recommendedthat it is loaded above 32MiB in order to avoid the need to relocateprior to decompression, which will make the boot process slightlyfaster.

When booting a raw (non-zImage) kernel the constraints are tighter.In this case the kernel must be loaded at an offset into system equalto TEXT_OFFSET - PAGE_OFFSET.

In any case, the following conditions must be met:

• Quiesce all DMA capable devices so that memory does not getcorrupted by bogus network packets or disk data. This will saveyou 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, orphysical 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, theCPU must be in SVC mode. (A special exception exists for Angel)

  CPUs which include support for the virtualization extensions can beentered in HYP mode in order to enable the kernel to make full use ofthese extensions. This is the recommended boot method for such CPUs,unless the virtualisations are already in use by a pre-installedhypervisor.

  If the kernel is not entered in HYP mode for any reason, it must beentered 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 tothe HYP mode configuration in addition to the ordinary PL1 (privilegedkernel modes) configuration. In addition, all traps into thehypervisor must be disabled, and PL1 access must be granted for allperipherals and CPU resources for which this is architecturallypossible. Except for entering in HYP mode, the system configurationshould be such that a kernel which does not include support for thevirtualization extensions can boot correctly without extra help.

• The boot loader is expected to call the kernel image by jumpingdirectly to the first instruction of the kernel image.

  On CPUs supporting the ARM instruction set, the entry must bemade in ARM state, even for a Thumb-2 kernel.

  On CPUs supporting only the Thumb instruction set such asCortex-M class CPUs, the entry must be made in Thumb state.