Documentation / arm / porting.rst

Based on kernel version 6.4.12. Page generated on 2023-08-29 08:47 EST.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137

Taken from list archive at

Initial definitions

The following symbol definitions rely on you knowing the translation that
__virt_to_phys() does for your machine.  This macro converts the passed
virtual address to a physical address.  Normally, it is simply:

		phys = virt - PAGE_OFFSET + PHYS_OFFSET

Decompressor Symbols

	Start address of decompressor.  There's no point in talking about
	virtual or physical addresses here, since the MMU will be off at
	the time when you call the decompressor code.  You normally call
	the kernel at this address to start it booting.  This doesn't have
	to be located in RAM, it can be in flash or other read-only or
	read-write addressable medium.

	Start address of zero-initialised work area for the decompressor.
	This must be pointing at RAM.  The decompressor will zero initialise
	this for you.  Again, the MMU will be off.

	This is the address where the decompressed kernel will be written,
	and eventually executed.  The following constraint must be valid:

		__virt_to_phys(TEXTADDR) == ZRELADDR

	The initial part of the kernel is carefully coded to be position

	Physical address to place the initial RAM disk.  Only relevant if
	you are using the bootpImage stuff (which only works on the old
	struct param_struct).

	Virtual address of the initial RAM disk.  The following  constraint
	must be valid:

		__virt_to_phys(INITRD_VIRT) == INITRD_PHYS

	Physical address of the struct param_struct or tag list, giving the
	kernel various parameters about its execution environment.

Kernel Symbols

	Physical start address of the first bank of RAM.

	Virtual start address of the first bank of RAM.  During the kernel
	boot phase, virtual address PAGE_OFFSET will be mapped to physical
	address PHYS_OFFSET, along with any other mappings you supply.
	This should be the same value as TASK_SIZE.

	The maximum size of a user process in bytes.  Since user space
	always starts at zero, this is the maximum address that a user
	process can access+1.  The user space stack grows down from this

	Any virtual address below TASK_SIZE is deemed to be user process
	area, and therefore managed dynamically on a process by process
	basis by the kernel.  I'll call this the user segment.

	Anything above TASK_SIZE is common to all processes.  I'll call
	this the kernel segment.

	(In other words, you can't put IO mappings below TASK_SIZE, and
	hence PAGE_OFFSET).

	Virtual start address of kernel, normally PAGE_OFFSET + 0x8000.
	This is where the kernel image ends up.  With the latest kernels,
	it must be located at 32768 bytes into a 128MB region.  Previous
	kernels placed a restriction of 256MB here.

	Virtual address for the kernel data segment.  Must not be defined
	when using the decompressor.

	Virtual addresses bounding the vmalloc() area.  There must not be
	any static mappings in this area; vmalloc will overwrite them.
	The addresses must also be in the kernel segment (see above).
	Normally, the vmalloc() area starts VMALLOC_OFFSET bytes above the
	last virtual RAM address (found using variable high_memory).

	Offset normally incorporated into VMALLOC_START to provide a hole
	between virtual RAM and the vmalloc area.  We do this to allow
	out of bounds memory accesses (eg, something writing off the end
	of the mapped memory map) to be caught.  Normally set to 8MB.

Architecture Specific Macros

	`pram` specifies the physical start address of RAM.  Must always
	be present, and should be the same as PHYS_OFFSET.

	`pio` is the physical address of an 8MB region containing IO for
	use with the debugging macros in arch/arm/kernel/debug-armv.S.

	`vio` is the virtual address of the 8MB debugging region.

	It is expected that the debugging region will be re-initialised
	by the architecture specific code later in the code (via the
	MAPIO function).

	Same as, and see PARAMS_PHYS.

	Machine specific fixups, run before memory subsystems have been

	Machine specific function to map IO areas (including the debug
	region above).

	Machine specific function to initialise interrupts.