[PATCH] arm: update advice on kernel and FDT load address.

[Russell King - ARM Linux]

On Tue, Jul 30, 2013 at 03:30:51PM +0100, Ian Campbell wrote:
> On Tue, 2013-07-30 at 15:04 +0100, Russell King - ARM Linux wrote:
> > On Tue, Jul 30, 2013 at 02:56:10PM +0100, Ian Campbell wrote:
> > > Actually I've just noticed that the existing text in
> > > Documentation/arm/Booting only talks about zImage and 32KiB, so perhaps
> > > not worrying about non-zImage is acceptable?
> > 
> > Straight binary kernels have tighter requirements, because they're _not_
> > entirely position independent.  Firstly, with dynamic physical offset,
> > they calculate that based on their position in physical space when
> > they're first called.
> > 
> > So, with a PAGE_OFFSET of 0xc0000000 and a link address of 0xc0008000,
> > a binary kernel image better be placed at 0xXXXX8000, and it _will_
> > assume that RAM starts 32KiB below itself.  And yes, the 32KiB offset
> > applies as the kernel uses TEXTADDR-16KiB to store the initial page
> > tables.
> 
> OK, so in the non-zImage case 32KiB is a Requirement and not just a
> recommendation. I'll update things to that affect.

Not quite a requirement.  The basic requirements for plain images are:

	kernel_offset = TEXT_OFFSET - PAGE_OFFSET
	assert(kernel_offset >= 32KiB)

	physical_memory = kernel_load_address - kernel_offset
	assert(IS_ALIGNED(physical_memory, 2MiB))

	for each memory node(node) {
		assert(physical_memory <= node->start_address);
	}

In other words, you can't link a kernel for 0xc0008000, and then load
it at (eg) 0xX000c000 or 0xX0004000 and expect it to work.