[PATCH] ARM: let CPUs not being able to run in ARM mode enter in THUMB mode

[Uwe Kleine-König]

Hello Dave,

On Tue, Jan 29, 2013 at 12:44:27PM +0000, Dave Martin wrote:
> On Fri, Jan 11, 2013 at 12:39:57PM +0100, Uwe Kleine-König wrote:
> > Some ARM cores are not capable to run in ARM mode (e.g. Cortex-M3). So
> > obviously these cannot enter the kernel in ARM mode. Make an exception
> > for them and let them enter in THUMB mode.
> > 
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
> > ---
> >  arch/arm/kernel/head-nommu.S |    8 +++++++-
> >  arch/arm/kernel/head.S       |    8 +++++++-
> >  arch/arm/mm/Kconfig          |    6 ++++++
> >  3 files changed, 20 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
> > index 3782320..ae7ed46 100644
> > --- a/arch/arm/kernel/head-nommu.S
> > +++ b/arch/arm/kernel/head-nommu.S
> > @@ -32,15 +32,21 @@
> >   * numbers for r1.
> >   *
> >   */
> > -	.arm
> >  
> >  	__HEAD
> > +
> > +#ifdef CONFIG_THUMBONLY_CPU
> > +	.thumb
> > +ENTRY(stext)
> > +#else
> > +	.arm
> >  ENTRY(stext)
> >  
> >   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:			)
> > +#endif
> 
> The behaviour is that we start the file is kernel entry state, then
> we switch to kernel run state.
> 
> The switch is only needed for kernels which run on CPUs supporting
> both states, where the run state is not ARM.
> 
> So, it would be neater for these entry sequences to look something
> like:
> 
> HAS_ARM(.arm)	@ because -mthumb is default for Thumb kernels anyway
> 
> ENTRY(stext)
> HAS_ARM(THUMB(	@ code to switch to Thumb ))
> 
> 	@ real code starts here, in run state.
> 
> 
> 
> Where
> 
> #ifdef CONFIG_THUMBONLY_CPU
> #define HAS_ARM(x...)
> #else
> #define HAS_ARM(x...) x
> #endif
> 
> (I haven't read the whole thread yet, so decisions about what
> config variables and macro names should be used may be overridden
> by other people...)
I don't agree on better readability, the result would look like:

		__HEAD

	HAS_ARM(.arm)

		ENTRY(stext)

	HAS_ARM(THUMB(adr	r9, BSYM(1f)	)
	HAS_ARM(THUMB(bx	r9		)
	HAS_ARM(THUMB(.thumb			)
	HAS_ARM(THUMB(1:			)

in contrast to:

		__HEAD

	#ifdef CONFIG_CPU_THUMBONLY
		.thumb
	ENTRY(stext)
	#else
		.arm
	ENTRY(stext)

	 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:                       )
	#endif

(modulo comments and indention maybe). Even though it uses an #ifdef I
consider the latter variant more readable.  Maybe it's only me? Other
than that I'd probably choose a longer name instead of HAS_ARM to make
it more self-documenting what it is about (something with _ISA_ in its
name). This obviously makes readability worse.

So I'm still happy with the version I sent.

Best regards
Uwe

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