diff options
Diffstat (limited to 'Documentation/arm/Booting')
| -rw-r--r-- | Documentation/arm/Booting | 64 |
1 files changed, 53 insertions, 11 deletions
diff --git a/Documentation/arm/Booting b/Documentation/arm/Booting index a341d87d276..371814a3671 100644 --- a/Documentation/arm/Booting +++ b/Documentation/arm/Booting @@ -18,7 +18,8 @@ following: 2. Initialise one serial port. 3. Detect the machine type. 4. Setup the kernel tagged list. -5. Call the kernel image. +5. Load initramfs. +6. Call the kernel image. 1. Setup and initialise RAM @@ -120,12 +121,27 @@ 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. The recommended placement is in the first 16KiB of RAM -with the caveat that it may not be located at physical address 0 since -the kernel interprets a value of 0 in r2 to mean neither a tagged list -nor a dtb were passed. +overwrite it, whilst remaining within the region which will be covered +by the kernel's low-memory mapping. -5. Calling the kernel image +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 @@ -136,11 +152,17 @@ 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 (at any location) and -called there. Note that the kernel uses 16K of RAM below the image -to store page tables. The recommended placement is 32KiB into RAM. +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 either case, the following conditions must be met: +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 @@ -154,13 +176,33 @@ In either case, the following conditions must be met: - CPU mode All forms of interrupts must be disabled (IRQs and FIQs) - The CPU must be in SVC mode. (A special exception exists for Angel) + + 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. |