[U-Boot] Skipping relocation RAM to RAM, esp. on i.MX6?

[Dirk Behme]

Hi,

on i.MX6 devices, e.g. ARM2 or SabreLite, the ROM boot loader copies the 
U-Boot image from the boot device, e.g. the SD card, to the main memory. 
This does mean that U-Boot is started in RAM.

With this, one might wonder why any relocation RAM -> RAM is done anyway 
and if this could be skipped?

Looking into the details shows that board_init_f() in 
arch/arm/lib/board.c and relocate_code() in arch/arm/cpu/armv7/start.S 
[1] are involved in this.

In board_init_f() the relocation destination address 'addr' is 
calculated. This is basically at the end of the available RAM (- some 
space for various stuff like TLB tables etc.). At SabreLite this results 
in 0x4FF8D000.

By the boot loader, the U-Boot is loaded to

CONFIG_SYS_TEXT_BASE		0x17800000

This results in relocate_code() copying U-Boot from RAM 0x17800000 to 
RAM 0x4FF8D000.

Setting CONFIG_SYS_TEXT_BASE to the relocation destination address 
0x4FF8D000 does avoid the (unnecessary?) copy by

cmp	r0, r6
moveq	r9, #0	/* no relocation. relocation offset(r9) = 0 */
beq	clear_bss /* skip relocation */

in relocate_code().

But:

1) The resulting image still runs without the relocation 
(CONFIG_SYS_TEXT_BASE 0x4FF8D000). But e.g. the U-Boot command line 
doesn't work properly any more. Most probably this is because not only 
the copy is skipped by the 'beq	clear_bss', but the whole 'fix .rel.dyn 
relocations' is skipped too.

2) It's hard to set CONFIG_SYS_TEXT_BASE at compile time to the 
relocation address calculated at runtime in board_init_f() due to the 
amount of #ifdef and runtime calculation done there. So finding a 
generic approach which could easily defined in the config files to avoid 
the relocation seems difficult.


I haven't tried it, but for (1) it might help to not jump to clear_bss, 
but instead jumping to the 'fix .rel.dyn relocations' section. Just 
avoiding the extra copy.

For (2) I don't have an idea how to solve this cleanly.


Do I have missed anything? Is there a clean way to skip the extra copy 
from RAM -> RAM in this case? Any idea or opinions?

Many thanks and best regards

Dirk

[1]

arch/arm/cpu/armv7/start.S:

relocate_code:
	mov	r4, r0	/* save addr_sp */
	mov	r5, r1	/* save addr of gd */
	mov	r6, r2	/* save addr of destination */

	/* Set up the stack						    */
stack_setup:
	mov	sp, r4

	adr	r0, _start
	cmp	r0, r6
	moveq	r9, #0		/* no relocation. relocation offset(r9) = 0 */
	beq	clear_bss		/* skip relocation */
	mov	r1, r6			/* r1 <- scratch for copy_loop */
	ldr	r3, _image_copy_end_ofs
	add	r2, r0, r3		/* r2 <- source end address	    */

copy_loop:
	ldmia	r0!, {r9-r10}		/* copy from source address [r0]    */
	stmia	r1!, {r9-r10}		/* copy to   target address [r1]    */
	cmp	r0, r2			/* until source end address [r2]    */
	blo	copy_loop

#ifndef CONFIG_SPL_BUILD
	/*
	 * fix .rel.dyn relocations
	 */
	ldr	r0, _TEXT_BASE		/* r0 <- Text base */
	sub	r9, r6, r0		/* r9 <- relocation offset */
	ldr	r10, _dynsym_start_ofs	/* r10 <- sym table ofs */
	add	r10, r10, r0		/* r10 <- sym table in FLASH */
	ldr	r2, _rel_dyn_start_ofs	/* r2 <- rel dyn start ofs */
	add	r2, r2, r0		/* r2 <- rel dyn start in FLASH */
	ldr	r3, _rel_dyn_end_ofs	/* r3 <- rel dyn end ofs */
	add	r3, r3, r0		/* r3 <- rel dyn end in FLASH */
fixloop:
	ldr	r0, [r2]		/* r0 <- location to fix up, IN FLASH! */
	add	r0, r0, r9		/* r0 <- location to fix up in RAM */
	ldr	r1, [r2, #4]
	and	r7, r1, #0xff
	cmp	r7, #23			/* relative fixup? */
	beq	fixrel
	cmp	r7, #2			/* absolute fixup? */
	beq	fixabs
	/* ignore unknown type of fixup */
	b	fixnext
fixabs:
	/* absolute fix: set location to (offset) symbol value */
	mov	r1, r1, LSR #4		/* r1 <- symbol index in .dynsym */
	add	r1, r10, r1		/* r1 <- address of symbol in table */
	ldr	r1, [r1, #4]		/* r1 <- symbol value */
	add	r1, r1, r9		/* r1 <- relocated sym addr */
	b	fixnext
fixrel:
	/* relative fix: increase location by offset */
	ldr	r1, [r0]
	add	r1, r1, r9
fixnext:
	str	r1, [r0]
	add	r2, r2, #8		/* each rel.dyn entry is 8 bytes */
	cmp	r2, r3
	blo	fixloop
	b	clear_bss
_rel_dyn_start_ofs:
	.word __rel_dyn_start - _start
_rel_dyn_end_ofs:
	.word __rel_dyn_end - _start
_dynsym_start_ofs:
	.word __dynsym_start - _start

#endif	/* #ifndef CONFIG_SPL_BUILD */

clear_bss:
	ldr	r0, _bss_start_ofs
	ldr	r1, _bss_end_ofs
	mov	r4, r6			/* reloc addr */
	add	r0, r0, r4
	add	r1, r1, r4
	mov	r2, #0x00000000		/* clear