[PATCH 1/3] ARM: debug: use kconfig choice for selecting DEBUG_LL UART

[Russell King - ARM Linux]

On Sun, Aug 21, 2011 at 08:18:35PM +0100, Russell King - ARM Linux wrote:
> On Sun, Aug 21, 2011 at 03:02:25PM -0400, Nicolas Pitre wrote:
> > Obviously the way to go is to use a .data variable.
> > 
> > And it can be referenced from assembly code in a position independent 
> > manner:
> > 
> >         .macro  get_ptr_pic, ptr, symbol
> >         b       199f
> >         .align
> > 198:    .long   .
> >         .long   \symbol
> > 199:    adr     \ptr, 198b              @ get relative address of 198b
> >         ldr     ip, [\ptr]              @ get absolute address of 198b
> >         sub     ip, ip, \ptr            @ offset between abs and rel
> >         ldr     \ptr, [\ptr, #4]        @ absolute address of \symbol
> >         sub     \ptr, \ptr, ip          @ make it relative
> >         .endm
> 
> And have you checked that you can crap over 'ip' in the early assembly
> code?  There are some (small) places where that register is used, which
> means you accidentally corrupt that register.

And further to this, I'll point out that the debugging functions are
*explicitly* designed to avoid corrupting any more than just r0-r3
and lr.  That's not just the IO functions but also the hex and string
printing functions.

And the head*.S code is explicitly written to expect r0-r3 to be
corrupted - which basically means that no long-term values are held in
those registers.