[RFC PATCH v3] ARM: Introduce patching of phys_to_virt and vice versa

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Wed, Nov 10, 2010 at 03:23:50PM -0500, Nicolas Pitre wrote:
> > 	add	rd, rn, #PHYS_OFFSET - PAGE_OFFSET
> > 
> > and phys_to_virt() is:
> > 
> > 	sub	rd, rn, #PHYS_OFFSET - PAGE_OFFSET
> 
> Does this work even if PHYS_OFFSET - PAGE_OFFSET goes negative? 

Yes.

	add	rd, rn, #0xc0000000
	sub	rd, rn, #0x40000000

are identical operations.  The operand is an 8 bit constant, zero padded,
rotated right by (in this case) 8 bits - there can be no sign extension.

> On Wed, 10 Nov 2010, Russell King - ARM Linux wrote:
> > Here's something which uses the above ideas (untested).  I think this is
> > something we can (and should) do unconditionally for the !XIP cases.
> 
> What do you mean by "unconditionally"?

For any machine where the v:p offset is respresentable.  It means that
we increase the amount of testing, and actually this becomes the
standard way.

> > +2:	cmp	r4, r5
> > +	ldrlo	r7, [r4], #4
> > +	ldrlo	ip, [r7, r3]
> > +	bic	ip, ip, #0x000000ff
> > +	bic	ip, ip, #0x00000f00
> > +	orr	ip, ip, r6
> 
> The above 3 insns could be replaced with:
> 
> 	mov	ip, ip, lsr #12
> 	orr	ip, r6, ip, lsl #12

They could be, but it's not worth optimizing to that extent - we're
already hitting load delays for both of those ldr instructions that
this isn't going to be blindingly fast... at least there aren't
thousands of them to fixup.