[linux-block.git] / Documentation / arm / porting.rst

=======
Porting
=======

Taken from list archive at http://lists.arm.linux.org.uk/pipermail/linux-arm-kernel/2001-July/004064.html

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
--------------------

ZTEXTADDR
	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.

ZBSSADDR
	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.

ZRELADDR
	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
	independent.

INITRD_PHYS
	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).

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

		__virt_to_phys(INITRD_VIRT) == INITRD_PHYS

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


Kernel Symbols
--------------

PHYS_OFFSET
	Physical start address of the first bank of RAM.

PAGE_OFFSET
	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.

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
	address.

	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).

TEXTADDR
	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.

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

VMALLOC_START / VMALLOC_END
	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).

VMALLOC_OFFSET
	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
----------------------------

BOOT_MEM(pram,pio,vio)
	`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).

BOOT_PARAMS
	Same as, and see PARAMS_PHYS.

FIXUP(func)
	Machine specific fixups, run before memory subsystems have been
	initialised.

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

INITIRQ(func)
	Machine specific function to initialise interrupts.
Commit	Line	Data
dc7a12bd MCC	1	=======
	2	Porting
	3	=======
	4
1da177e4 LT	5	Taken from list archive at http://lists.arm.linux.org.uk/pipermail/linux-arm-kernel/2001-July/004064.html
	6
	7	Initial definitions
	8	-------------------
	9
	10	The following symbol definitions rely on you knowing the translation that
	11	__virt_to_phys() does for your machine. This macro converts the passed
	12	virtual address to a physical address. Normally, it is simply:
	13
	14	phys = virt - PAGE_OFFSET + PHYS_OFFSET
	15
	16
	17	Decompressor Symbols
	18	--------------------
	19
	20	ZTEXTADDR
	21	Start address of decompressor. There's no point in talking about
	22	virtual or physical addresses here, since the MMU will be off at
	23	the time when you call the decompressor code. You normally call
	24	the kernel at this address to start it booting. This doesn't have
	25	to be located in RAM, it can be in flash or other read-only or
	26	read-write addressable medium.
	27
	28	ZBSSADDR
	29	Start address of zero-initialised work area for the decompressor.
	30	This must be pointing at RAM. The decompressor will zero initialise
	31	this for you. Again, the MMU will be off.
	32
	33	ZRELADDR
	34	This is the address where the decompressed kernel will be written,
	35	and eventually executed. The following constraint must be valid:
	36
	37	__virt_to_phys(TEXTADDR) == ZRELADDR
	38
	39	The initial part of the kernel is carefully coded to be position
	40	independent.
	41
	42	INITRD_PHYS
	43	Physical address to place the initial RAM disk. Only relevant if
	44	you are using the bootpImage stuff (which only works on the old
	45	struct param_struct).
	46
	47	INITRD_VIRT
	48	Virtual address of the initial RAM disk. The following constraint
	49	must be valid:
	50
	51	__virt_to_phys(INITRD_VIRT) == INITRD_PHYS
	52
	53	PARAMS_PHYS
	54	Physical address of the struct param_struct or tag list, giving the
	55	kernel various parameters about its execution environment.
	56
	57
	58	Kernel Symbols
	59	--------------
	60
	61	PHYS_OFFSET
	62	Physical start address of the first bank of RAM.
	63
	64	PAGE_OFFSET
	65	Virtual start address of the first bank of RAM. During the kernel
	66	boot phase, virtual address PAGE_OFFSET will be mapped to physical
	67	address PHYS_OFFSET, along with any other mappings you supply.
	68	This should be the same value as TASK_SIZE.
69
70	TASK_SIZE
71	The maximum size of a user process in bytes. Since user space
72	always starts at zero, this is the maximum address that a user
73	process can access+1. The user space stack grows down from this
74	address.
75
76	Any virtual address below TASK_SIZE is deemed to be user process
77	area, and therefore managed dynamically on a process by process
78	basis by the kernel. I'll call this the user segment.
79
80	Anything above TASK_SIZE is common to all processes. I'll call
81	this the kernel segment.
82
83	(In other words, you can't put IO mappings below TASK_SIZE, and
84	hence PAGE_OFFSET).
85
86	TEXTADDR
87	Virtual start address of kernel, normally PAGE_OFFSET + 0x8000.
88	This is where the kernel image ends up. With the latest kernels,
89	it must be located at 32768 bytes into a 128MB region. Previous
90	kernels placed a restriction of 256MB here.
91
92	DATAADDR
93	Virtual address for the kernel data segment. Must not be defined
94	when using the decompressor.
95
dc7a12bd	96	VMALLOC_START / VMALLOC_END
1da177e4 LT	97	Virtual addresses bounding the vmalloc() area. There must not be
	98	any static mappings in this area; vmalloc will overwrite them.
	99	The addresses must also be in the kernel segment (see above).
	100	Normally, the vmalloc() area starts VMALLOC_OFFSET bytes above the
	101	last virtual RAM address (found using variable high_memory).
	102
	103	VMALLOC_OFFSET
	104	Offset normally incorporated into VMALLOC_START to provide a hole
	105	between virtual RAM and the vmalloc area. We do this to allow
	106	out of bounds memory accesses (eg, something writing off the end
	107	of the mapped memory map) to be caught. Normally set to 8MB.
	108
	109	Architecture Specific Macros
	110	----------------------------
	111
	112	BOOT_MEM(pram,pio,vio)
dc7a12bd	113	`pram` specifies the physical start address of RAM. Must always
1da177e4 LT	114	be present, and should be the same as PHYS_OFFSET.
1da177e4 LT	115
dc7a12bd	116	`pio` is the physical address of an 8MB region containing IO for
1da177e4 LT	117	use with the debugging macros in arch/arm/kernel/debug-armv.S.
1da177e4 LT	118
dc7a12bd	119	`vio` is the virtual address of the 8MB debugging region.
1da177e4 LT	120
	121	It is expected that the debugging region will be re-initialised
	122	by the architecture specific code later in the code (via the
	123	MAPIO function).
	124
	125	BOOT_PARAMS
	126	Same as, and see PARAMS_PHYS.
	127
	128	FIXUP(func)
	129	Machine specific fixups, run before memory subsystems have been
	130	initialised.
	131
	132	MAPIO(func)
	133	Machine specific function to map IO areas (including the debug
	134	region above).
	135
	136	INITIRQ(func)
	137	Machine specific function to initialise interrupts.