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diff --git a/Documentation/arm/Booting b/Documentation/arm/Booting deleted file mode 100644 index f1f965ce93d6..000000000000 --- a/Documentation/arm/Booting +++ /dev/null @@ -1,218 +0,0 @@ - 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. |