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

[dave.martin@linaro.org (Dave Martin)]

On Thu, Jan 31, 2013 at 04:51:59PM +0100, Uwe Kleine-K?nig wrote:
> 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-K?nig 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-K?nig <u.kleine-koenig@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.

Well, maybe so.

Note that Thumb is also the default instruction set for any Thumb-2
kernel; that's why we needed .arm in the first place.


We could turn the switch to Thumb into a macro in assembler.h -- that's
actually needed elsewhere:

#if defined(CONFIG_THUMB2_KERNEL) && !defined(CONFIG_CPU_THUMBONLY)

.macro entry_isa
	.arm
.endm

.macro switch_to_kernel_isa rtemp
 THUMB(		adr	\rtemp, BSYM(9999f)	)
 THUMB(		bx	\rtemp			)
 THUMB(		.thumb				)
 THUMB(	9999:					)
.endm

#else

.macro entry_isa
.endm
.macro switch_to_kernel_isa rtemp
.endm

#endif


Then head.S becomes

	__HEAD

	entry_isa

ENTRY(stext)
	switch_to_kernel_isa r9



The macros can be reused in the zImage decompressor, the mcpm low-level
entry point and possibly elsewhere.

What do you think?

Cheers
---Dave