kernel entry for thumb2-only cpus

[Nicolas Pitre]

On Sun, 5 Aug 2012, Uwe Kleine-König wrote:

> 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-König 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:

Let's stop splitting hairs.

A Cortex-M kernel simply won't run on anything else.  It certainly won't 
run on ARM mode capable processors.  So in this case just make the 
kernel entry point Thumb mode only by #ifdef'ing out the .arm part.

This is good _only_ for CPUs without any ARM mode capability though.


Nicolas