[PATCH 5/5] ARM: Cortex-M3: Add support for exception handling

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

On Fri, Feb 24, 2012 at 10:12:06PM +0000, Catalin Marinas wrote:
> Hi Uwe,
> 
> 2012/2/24 Uwe Kleine-K?nig <u.kleine-koenig@pengutronix.de>:
> > On Thu, Feb 16, 2012 at 10:20:02PM +0000, Russell King - ARM Linux wrote:
> >> On Thu, Feb 16, 2012 at 09:18:10PM +0100, Uwe Kleine-K?nig wrote:
> >> > + ? .macro ?v7m_exception_entry
> >> > + ? cpsid ? i
> >> > + ? tst ? ? lr, #0x8 ? ? ? ? ? ? ? ?@ check the return stack
> >> > + ? bne ? ? 1f ? ? ? ? ? ? ? ? ? ? ?@ exception on process stack
> >> > + ? add ? ? r12, sp, #32 ? ? ? ? ? ?@ MSP before exception
> >> > + ? stmdb ? sp!, {r4-r12, lr} ? ? ? @ push unsaved registers
> >> > + ? b ? ? ? 2f
> >> > +1:
> >> > + ? mrs ? ? r12, psp ? ? ? ? ? ? ? ?@ get the process stack
> >> > + ? sub ? ? sp, #S_FRAME_SIZE
> >> > + ? stmia ? sp, {r4-r12, lr} ? ? ? ?@ push unsaved registers
> >> > + ? ldmia ? r12, {r0-r3, r6, r8-r10} @ load automatically saved registers
> >> > + ? add ? ? r12, sp, #S_R0
> >> > + ? stmia ? r12, {r0-r3, r6, r8-r10} @ fill in the rest of struct pt_regs
> >>
> >> I guess this means that pt_regs no longer contains r0..pc, cpsr, old_r0
> >> on this Cortex-M ?
> 
> Just to reply to Russell - yes, this part needs changing (could be the
> pt_regs saving or just the signal and trace code that copies them to
> user so that we preserve the ABI).
> 
> > I stared at the code now for some time and I wonder if it wouldn't be
> > the most nice solution to just do something like this on exception
> > entry:
> >
> > ? ? ? ?cpsid ? i
> > ? ? ? ?sub ? ? sp, #S_FRAME_SIZE
> > ? ? ? ?stmia ? sp, {r0-r12}
> > ? ? ? ?put_the_right_sp_to_sp[13]
> > ? ? ? ?put_lr_returnaddr_and_xPSR_from_right_stack_to_sp[14-16]
> >
> > For returning you could just do:
> >
> > ? ? ? ?add ? ? sp, #S_FRAME_SIZE
> > ? ? ? ?cpsie ? i
> > ? ? ? ?bx ? ? ?lr
> 
> For this kind of returning, do we actually change between Process and
> Handler stack?
According to ARMARM-v7-M when pc is written to 0xfXXXXXXX (with X
denoting don't care) also the stack is restored.

> But we still need to set the pt_regs to the user stack. The kernel
> would touch all of them if a signal is to be delivered (actually
> restoring them when returning from the signal handler).
Ah, I thought only r0 might be changed. I will bear that in mind.

Thanks for your insights
Uwe

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