Re: [PATCH 3/3] ARM: tegra: make sure the pointer on 4 byte align when THUMB2_KERNEL enabled

[Dave Martin <dave.martin-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On Tue, Apr 16, 2013 at 10:20:11AM -0600, Stephen Warren wrote:
> On 04/16/2013 08:13 AM, Dave Martin wrote:
> > On Mon, Apr 15, 2013 at 04:50:55PM -0600, Stephen Warren wrote:
> >> From: Joseph Lo <josephl-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> >>
> >> When building kernel with CONFIG_THUMB2_KERNEL, the data pointer in the
> >> assembly may not on the 4 byte alignment. Then causing a data abort when
> >> accessing the pointer. This patch add a ".align" flag in the head of the
> >> pointer. And always using 32-bit ADR Thumb instruction to make sure it
> >> won't build failure.
> >>
> >> Signed-off-by: Joseph Lo <josephl-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> >> Signed-off-by: Stephen Warren <swarren-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> >> ---
> >>  arch/arm/mach-tegra/reset-handler.S |    1 +
> >>  arch/arm/mach-tegra/sleep.h         |    3 ++-
> >>  2 files changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm/mach-tegra/reset-handler.S b/arch/arm/mach-tegra/reset-handler.S
> >> index e6de88a..519a8c5 100644
> >> --- a/arch/arm/mach-tegra/reset-handler.S
> >> +++ b/arch/arm/mach-tegra/reset-handler.S
> >> @@ -83,6 +83,7 @@ ENDPROC(tegra_resume)
> >>  
> >>  #ifdef CONFIG_CACHE_L2X0
> >>  	.globl	l2x0_saved_regs_addr
> >> +	.align
> >>  l2x0_saved_regs_addr:
> >>  	.long	0
> >>  #endif
> >> diff --git a/arch/arm/mach-tegra/sleep.h b/arch/arm/mach-tegra/sleep.h
> >> index 4ffae54..8e9b6af 100644
> >> --- a/arch/arm/mach-tegra/sleep.h
> >> +++ b/arch/arm/mach-tegra/sleep.h
> >> @@ -92,7 +92,8 @@
> >>  
> >>  #ifdef CONFIG_CACHE_L2X0
> >>  .macro l2_cache_resume, tmp1, tmp2, tmp3, phys_l2x0_saved_regs
> >> -	adr	\tmp1, \phys_l2x0_saved_regs
> >> + ARM(	adr	\tmp1, \phys_l2x0_saved_regs )
> >> + THUMB(	adr.w	\tmp1, \phys_l2x0_saved_regs )
> > 
> > Can you give an example of the assembler error you get without this?
> 
> arch/arm/mach-tegra/reset-handler.S: Assembler messages:
> arch/arm/mach-tegra/reset-handler.S:78: Error: invalid immediate for
> address calculation (value = 0x00000004)

The immediate in the 16-bit form of ADR is alignment-sensitive, and
can't address anything that's not on a word boundary.  What if you just
have the .align, without adr.w?

> This is with gcc-4.5.3, as:
> GNU assembler (crosstool-NG hg_unknown-Y8jGNvr6ygZCnf0dxYVdog@public.gmane.org) 2.20.1.20100303
> 
> > The target symbol is local and the assembler can see where it is,
> > so it should choose the correct variant for the adr instruction
> > with no need for the ".w" suffix.
> > 
> > If not, it could mean that there is a bug in the version of the
> > assembler you're using.
> 
> True. Switching to the Ubuntu-packaged ARM cross-compiler that ships
> with Ubuntu 12.10, I do not see this problem.
> 
> Still, many people probably still use older compilers.
> 
> > If it's definitely needed, you can append a Thumb-only .w suffix with
> > 
> > 	W(adr)	\tmp1, \phys_l2x0_saved_regs
> > 
> > ...which is slightly neater.
> 
> Yes, that certainly works too, even without the .align change in
> reset-handler.S.

Sure, although the unaligned access is not such a good idea anyway.

Keeping the adr fix is harmless, I just wanted to understand why it's
needed.

Cheers
---Dave