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

[Uwe Kleine-König]

On Fri, Feb 24, 2012 at 10:12:06PM +0000, Catalin Marinas wrote:
> Hi Uwe,
> 
> 2012/2/24 Uwe Kleine-König <u.kleine-koenig at 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-König 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-König            |
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