[PATCH] arm: ARMv4 assembly compatibility

[Andre Przywara]

On Wed, 10 Aug 2022 12:04:46 +0300
Sergei Antonov <saproj at gmail.com> wrote:

Hi,

> There is currently a problem that U-Boot can not work on ARMv4
> because assembly imlementations of memcpy() and some other functions
> use "bx lr" instruction that is not available on ARMv4 ("mov pc, lr"
> should be used instead).

So there is a comment in the current code, right above what's shown here in
the first hunk, which says:
/*
 * We only support cores that support at least Thumb-1 and thus we use
 * 'bx lr'
 */
This was added by Tom in commit 431afb4ef9fe, when he removed some code
very close to what you are adding back now.

So what is the story here? This commit seems to suggest U-Boot doesn't support
even ARMv5 without "T", has this changed? There are probably other code
places which would need adjustment to run on ARMv4?

Tom, can you say why support for Thumb-less code / older architectures was
dropped? No users, I guess?

Sergei, can you say what is this for? Are you adding support for a SoC
with an ARM810 core from 1996?

Cheers,
Andre



> 
> A working preprocessor-based solution to this problem is found in
> arch/arm/lib/relocate.S. Move it to the "ret" macro in
> arch/arm/include/asm/assembler.h and change all "bx lr" code
> to "ret lr" in functions that may run on ARMv4. Linux source code
> deals with this problem in the same manner.
> 
> Signed-off-by: Sergei Antonov <saproj at gmail.com>
> CC: Samuel Holland <samuel at sholland.org>
> CC: Ye Li <ye.li at nxp.com>
> CC: Simon Glass <sjg at chromium.org>
> CC: Andre Przywara <andre.przywara at arm.com>
> CC: Marek Vasut <marex at denx.de>
> CC: Sean Anderson <sean.anderson at seco.com>
> CC: Tom Rini <trini at konsulko.com>
> CC: Wolfgang Denk <wd at denx.de>
> ---
>  arch/arm/include/asm/assembler.h |  6 ++++++
>  arch/arm/lib/lib1funcs.S         |  8 ++++----
>  arch/arm/lib/memcpy.S            |  6 +++---
>  arch/arm/lib/relocate.S          | 10 ++--------
>  arch/arm/lib/setjmp.S            |  4 ++--
>  5 files changed, 17 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index b146918586..911f7a1b49 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -63,11 +63,17 @@
>   */
>  	.irp	c,,eq,ne,cs,cc,mi,pl,vs,vc,hi,ls,ge,lt,gt,le,hs,lo
>  	.macro	ret\c, reg
> +
> +	/* ARMv4- don't know bx lr but the assembler fails to see that */
> +#ifdef __ARM_ARCH_4__
> +	mov\c	pc, \reg
> +#else
>  	.ifeqs	"\reg", "lr"
>  	bx\c	\reg
>  	.else
>  	mov\c	pc, \reg
>  	.endif
> +#endif
>  	.endm
>  	.endr
>  
> diff --git a/arch/arm/lib/lib1funcs.S b/arch/arm/lib/lib1funcs.S
> index 700eee5fbb..7ff4446dd6 100644
> --- a/arch/arm/lib/lib1funcs.S
> +++ b/arch/arm/lib/lib1funcs.S
> @@ -377,7 +377,7 @@ ENTRY(__gnu_thumb1_case_sqi)
>  	lsls	r1, r1, #1
>  	add	lr, lr, r1
>  	pop	{r1}
> -	bx	lr
> +	ret	lr
>  ENDPROC(__gnu_thumb1_case_sqi)
>  .popsection
>  
> @@ -391,7 +391,7 @@ ENTRY(__gnu_thumb1_case_uqi)
>  	lsls	r1, r1, #1
>  	add	lr, lr, r1
>  	pop	{r1}
> -	bx	lr
> +	ret	lr
>  ENDPROC(__gnu_thumb1_case_uqi)
>  .popsection
>  
> @@ -406,7 +406,7 @@ ENTRY(__gnu_thumb1_case_shi)
>  	lsls	r1, r1, #1
>  	add	lr, lr, r1
>  	pop	{r0, r1}
> -	bx	lr
> +	ret	lr
>  ENDPROC(__gnu_thumb1_case_shi)
>  .popsection
>  
> @@ -421,7 +421,7 @@ ENTRY(__gnu_thumb1_case_uhi)
>  	lsls	r1, r1, #1
>  	add	lr, lr, r1
>  	pop	{r0, r1}
> -	bx	lr
> +	ret	lr
>  ENDPROC(__gnu_thumb1_case_uhi)
>  .popsection
>  #endif
> diff --git a/arch/arm/lib/memcpy.S b/arch/arm/lib/memcpy.S
> index eee7a219ce..a1c996f94e 100644
> --- a/arch/arm/lib/memcpy.S
> +++ b/arch/arm/lib/memcpy.S
> @@ -59,7 +59,7 @@
>  #endif
>  ENTRY(memcpy)
>  		cmp	r0, r1
> -		bxeq	lr
> +		reteq	lr
>  
>  		enter	r4, lr
>  
> @@ -148,7 +148,7 @@ ENTRY(memcpy)
>  		str1b	r0, ip, cs, abort=21f
>  
>  		exit	r4, lr
> -		bx	lr
> +		ret	lr
>  
>  9:		rsb	ip, ip, #4
>  		cmp	ip, #2
> @@ -258,7 +258,7 @@ ENTRY(memcpy)
>  
>  	.macro	copy_abort_end
>  	ldmfd	sp!, {r4, lr}
> -	bx	lr
> +	ret	lr
>  	.endm
>  
>  ENDPROC(memcpy)
> diff --git a/arch/arm/lib/relocate.S b/arch/arm/lib/relocate.S
> index 5102bfabde..dd6f2e3bd5 100644
> --- a/arch/arm/lib/relocate.S
> +++ b/arch/arm/lib/relocate.S
> @@ -61,7 +61,7 @@ ENTRY(relocate_vectors)
>  	stmia	r1!, {r2-r8,r10}
>  #endif
>  #endif
> -	bx	lr
> +	ret	lr
>  
>  ENDPROC(relocate_vectors)
>  
> @@ -127,13 +127,7 @@ relocate_done:
>  	mcr	p15, 0, r0, c7, c10, 4	/* drain write buffer */
>  #endif
>  
> -	/* ARMv4- don't know bx lr but the assembler fails to see that */
> -
> -#ifdef __ARM_ARCH_4__
> -	mov	pc, lr
> -#else
> -	bx	lr
> -#endif
> +	ret	lr
>  
>  ENDPROC(relocate_code)
>  
> diff --git a/arch/arm/lib/setjmp.S b/arch/arm/lib/setjmp.S
> index 176a1d5315..2f041aeef0 100644
> --- a/arch/arm/lib/setjmp.S
> +++ b/arch/arm/lib/setjmp.S
> @@ -17,7 +17,7 @@ ENTRY(setjmp)
>  	mov  ip, sp
>  	stm  a1, {v1-v8, ip, lr}
>  	mov  a1, #0
> -	bx   lr
> +	ret  lr
>  ENDPROC(setjmp)
>  .popsection
>  
> @@ -31,6 +31,6 @@ ENTRY(longjmp)
>  	bne  1f
>  	mov  a1, #1
>  1:
> -	bx   lr
> +	ret  lr
>  ENDPROC(longjmp)
>  .popsection