[GIT PULL] pxa: features for next

[Russell King - ARM Linux]

On Mon, Jul 11, 2011 at 04:31:04PM +0800, Eric Miao wrote:
> On Mon, Jul 11, 2011 at 3:52 PM, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> > On Mon, Jul 11, 2011 at 03:47:27PM +0800, Eric Miao wrote:
> >> On Mon, Jul 11, 2011 at 3:46 PM, Russell King - ARM Linux
> >> <linux at arm.linux.org.uk> wrote:
> >> > On Mon, Jul 11, 2011 at 03:44:54PM +0800, Eric Miao wrote:
> >> >> On Mon, Jul 11, 2011 at 3:41 PM, Russell King - ARM Linux
> >> >> <linux at arm.linux.org.uk> wrote:
> >> >> > On Mon, Jul 11, 2011 at 03:27:15PM +0800, Eric Miao wrote:
> >> >> >>       ARM: pxa: avoid accessing interrupt registers directly
> >> >> >>       ARM: pxa: introduce {icip,ichp}_handle_irq() to prepare MULTI_IRQ_HANDLER
> >> >> >
> >> >> > What happened about the __exception issue with asm_do_IRQ?
> >> >> >
> >> >>
> >> >> I just removed the __exception from the C handler.
> >> >
> >> > From asm_do_IRQ ?
> >> >
> >>
> >> No. From icip_handle_irq() and ichp_handle_irq(). Thought the ability to
> >> unwind asm_do_IRQ() is more important.
> >
> > Which means you didn't understand my objection when I reviewed your patch.
> >
> > The __exception annotation on a function causes this to happen:
> >
> > [<c002406c>] (asm_do_IRQ+0x6c/0x8c) from [<c0024b84>] (__irq_svc+0x44/0xcc)
> > Exception stack(0xc3897c78 to 0xc3897cc0)
> > 7c60:                                                       4022d320 4022e000
> > 7c80: 08000075 00001000 c32273c0 c03ce1c0 c2b49b78 4022d000 c2b420b4 00000001
> > 7ca0: 00000000 c3897cfc 00000000 c3897cc0 c00afc54 c002edd8 00000013 ffffffff
> >
> > Where that stack dump represents the pt_regs for the exception which
> > happened.  Any function found in while unwinding will cause this to
> > be printed.
> >
> > If you insert a C function between the IRQ assembly and asm_do_IRQ, the
> > dump you get from asm_do_IRQ will be the stack for your function, not
> > the pt_regs.  That makes the feature useless.
> >
> 
> Sorry for my stupidity, but I think I still don't get it quite correctly.
> When both functions are prefixed with __exception_irq_entry, if
> unwind works correctly, both stacks will be dumped, how would
> the asm_do_IRQ will be stack for the function inserted?

When __irq_svc - or any of the other exception handling assembly code -
calls the C code, the stack pointer will be pointing at the pt_regs
structure.

All the entry points into C code from the exception handling code are
marked with __exception or __exception_irq_enter to indicate that they
are one of the functions which has pt_regs above them.

Normally, when you've entered asm_do_IRQ() you will have this stack
layout (higher address towards top):

	pt_regs
	asm_do_IRQ frame

If you insert a C function between the exception assembly code and
asm_do_IRQ, you end up with this stack layout instead:

	pt_regs
	your function frame
	asm_do_IRQ frame

This means when we unwind, we'll get to asm_do_IRQ, and rather than
dumping out the pt_regs, we'll dump out your functions stack frame
instead, because that's what is above the asm_do_IRQ stack frame
rather than the expected pt_regs structure.