Re: get_irq_regs() from soft IRQ

[Jean Pihet <jpihet@mvista.com>]

On Monday 29 June 2009 17:19:31 Russell King - ARM Linux wrote:
> On Mon, Jun 29, 2009 at 04:31:18PM +0200, Jean Pihet wrote:
> > I am trying to get the latest IRQ registers from a timer or a work queue
> > but I am running into problems:
> > - get_irq_regs() returns NULL in some cases,
>
> It will always return NULL outside of IRQ context - and only returns valid
> pointers when used inside IRQ context.
Ok got it.

> It's one of these things that nests itself - when you have several IRQs
> being processed on one CPU, there are several register contexts saved,
> and get_irq_regs() returns the most recent one.
>
> > The use case is that the performance unit (PMNC) of the Cortex A8 has
> > some serious bug, in short the performance counters overflow IRQ is to be
> > avoided.
>
> I don't follow.  None of the PMNC support code in the mainline kernel
> uses get_irq_regs() outside of IRQ context.
That is correct. The Cortex A8 needs some special treatment.
The errata says that if the counters are overflowing at the same time as a 
coprocessor access is performed, the perf unit gets reset and/or locks up. In 
short the counters overflow is to be avoided and so the PMNC IRQ.

> > Some questions:
> > - is there a way to get the last 'real' IRQ registers from a timer or
> > work queue handler?
>
> No.  Outside of IRQ events, the saved IRQ context does not exist.
Ok. I wonder how to implement it correctly from here.
The ultimate goal is to feed the registers to oprofile for statistics 
gathering (mostly the PC). I do not see much benefit from oprofile without 
the PC statistics.

Thanks,
Jean