Re: About TEXTADDR, ZTEXTADDR, PAGE_OFFSET etc...

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Author: Russell King - ARM Linux
Date:  
To: Peter van Sebille
CC: linux-arm-kernel
Subject: Re: About TEXTADDR, ZTEXTADDR, PAGE_OFFSET etc...
Actually, I'll do it as a reply, and link it from the mailing list archive.

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.