[PATCH 3/5] omap: Add support for CONFIG_AUTO_ZRELADDR for DEBUG_LL

On Fri, 4 Feb 2011, Tony Lindgren wrote:

* Nicolas Pitre [off-list ref] [110203 19:32]:

quoted

On Thu, 3 Feb 2011, Tony Lindgren wrote:

quoted

This way we can have the debug-macro.S be common for omap1 and omap2+
and get sensible error messages booting the wrong zImage with
CONFIG_AUTO_ZRELADDR selected. Note that this does not seem to work
with u-boot and uImage.

...
 

quoted

Gaaaaah!  This is horrible.

-ENOREGISTERS :)
 

quoted

OK, you do want to store data in memory, right?  So the best way to 
always be position independent, even cross section, is to do the 
following (inspired by RMK's trick in the p2v series):

	.data

foo:	.word	0	@ value for sput
	.word	0	@ value for nik

	.text

bar:	.word	.
	.word	foo

sputnik:

	adr	r0, bar		@ get relative address of bar
	ldr	r1, [r0]	@ get absolute address of bar
	sub	r1, r1, r0	@ difference between abs and rel address
	ldr	r0, [r0, #4]	@ get absolute address of foo
	sub	r0, r0, r1	@ turn that into a relative address

	ldmia	r0, {r1, r2}	@ retrieve sput and nik
	add	r0, r1, r2
	mov	pc, lr

This code is totally position independent, so it works whether or not 
the MMU is active.  Therefore a similar technique in your macro (you 
could even put it into a macro for it) would allow you to get rid of the 
mrc and test for MMU active, get rid of the #ifdef, and not be limited 
to a 128 MB mask, etc.  And it is just one word longer than your 
shortest version i.e. 2 constants + 5 instructions vs 4 instructions + 2 
literal pool entries.

I think if we keep the address of the memory location in rx instead of
the port address value, then this could be done.

That would limit all the trickery to inituart only. Then addruart and
busyuart could just do ldr from the memory location to get the port
data without any trickery.

Just create a get_config_ptr macro or similar and the trickery will be 
nicely encapsulated.  You'd have:

	.macro get_config_ptr ptr, tmp
	b	9002f
	.align
9001:	.long	.
	.long	uart_param_storage
9002:	adr	\ptr, 9001b
	ldr	\tmp, [\ptr]
	sub	\tmp, \tmp, \ptr
	ldr	\ptr, [\ptr, #4]
	sub	\ptr, \ptr, \tmp
	.endm

	.macro addruart rp, rv
	get_config_ptr \rv
	ldr	\rp, [\rv]
	ldr	\rv, [\rv, #4]
	.endm

And for the other macros, you get the hardware address to use already, 
but you can still use get_config_ptr for extra config values you 
need if necessary.

In that case the mapping of IO space for the serial port in head.S
would have to be changed as that relies on rx containing the port
IO address..

Yes, and I'm not sure if it is worth changing that at this point when it 
is possible to do it without introducing such a change which would 
affect all machines again.


Nicolas