kernel entry for thumb2-only cpus

[u.kleine-koenig@pengutronix.de (Uwe Kleine-König)]

On Fri, Aug 03, 2012 at 12:45:32PM +0100, Will Deacon wrote:
> On Fri, Aug 03, 2012 at 10:04:52AM +0100, Uwe Kleine-K?nig wrote:
> > Hello,
> 
> Hi Uwe,
> 
> > in both arch/arm/kernel/head.S and arch/arm/kernel/head-nommu.S we have
> > 
> > 	.arm
> >  THUMB( adr     r9, BSYM(1f)    )       @ Kernel is always entered in ARM.
> >  THUMB( bx      r9              )       @ If this is a Thumb-2 kernel,
> >  THUMB( .thumb                  )       @ switch to Thumb now.
> >  THUMB(1:                       )
> > 
> > as first instructions at the entry point. This is a problem for
> > thumb2-only cpus (e.g. Cortex-M3).
> 
> Yup, Documentation/arm/Booting mentions this too.
> 
> > Up to now I commented out the first three lines for the Cortex-M3 port.
> > What should we do about that. There are two possibilities I see:
> > 
> >  a) introduce a kconfig symbol for thumb2-only builds and #ifdef the
> >     first three lines out if it is given.
> >  b) expect the bootloader to directly jump to the label 1.
> > 
> > The downside of a) is that a boot loader on a cpu that is capable to
> > execute the tradtional instructions would need to detect if the switch
> > to thumb is there or not and react accordingly. (In fact it needs to
> > distringuish three cases:
> > 
> >  - traditional kernel
> >  - thumb2 kernel with ARM entry
> >  - thumb2 kernel without ARM entry
> > 
> > .) So I think b) is the more sensible option in the long run.
> > 
> > What do you think?
> 
> How about something like:
> 
> 
> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> index 835898e..9f07be2 100644
> --- a/arch/arm/kernel/head.S
> +++ b/arch/arm/kernel/head.S
> @@ -86,6 +86,7 @@
>         __HEAD
>  ENTRY(stext)
>  
> + THUMB(        .inst   0xe200e004      )       @ ARM: and lr, r0, #4 T2: b 1f
>   THUMB(        adr     r9, BSYM(1f)    )       @ Kernel is always entered in ARM.
>   THUMB(        bx      r9              )       @ If this is a Thumb-2 kernel,
>   THUMB(        .thumb                  )       @ switch to Thumb now.
Great idea, but it doesn't work as suggested. My compiler already barfs
on the .arm above:

	arch/arm/kernel/head-nommu.S:36: Error: selected processor does not support ARM opcodes
	arch/arm/kernel/head-nommu.S:42: Error: attempt to use an ARM instruction on a Thumb-only processor -- `adr r9,1f+1'
	arch/arm/kernel/head-nommu.S:43: Error: attempt to use an ARM instruction on a Thumb-only processor -- `bx r9'

but something like that should be doable.

Best regards
Uwe


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Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |