Re: [PATCH 3.18-rc3 v9 5/5] arm: smp: Handle ipi_cpu_backtrace() using FIQ (if available)

From: Russell King - ARM Linux
Date: Mon Dec 01 2014 - 05:38:51 EST

On Mon, Dec 01, 2014 at 11:32:00AM +0100, Tim Sander wrote:
> Hi Russel, Daniel
> Am Freitag, 28. November 2014, 10:08:28 schrieb Russell King - ARM Linux:
> > The two things are mutually exclusive. You can either have FIQ being used
> > for debug purposes, where we decode the FIQ reason and call some function
> > (which means that we will only service one FIQ at a time) or you can use
> > it in exclusive mode (provided by fiq.c) where your handler has sole usage
> > of the vector, and benefits from fast and immediate servicing of the event.
> As far as i am aware, die CONFIG_FIQ symbol is not pulled by all ARM
> platforms. Since there are ARM platforms which don't use this symbol but the
> hardware is fully capable of handling FIQ requests i would expect, that adding
> CONFIG_FIQ to a plattform, that this platform honors the set_fiq_handler
> functionality.

That whole paragraph doesn't make much sense to me.

Look, in my mind it is very simple. If you are using CONFIG_FIQ on a
SMP platform, your life will be very difficult. The FIQ code enabled
by that symbol is not designed to be used on SMP systems, *period*.

If you decide to enable CONFIG_FIQ, and you use that code on a SMP
platform, I'm going to say right now so it's totally clear: if you
encounter a problem, I don't want to know about it. The code is not
designed for use on that situation.

Therefore, as far as I'm concerned, the two facilities are mututally

I had thought about whether the IPI FIQ should be disabled when a
replacement FIQ handler is installed, I deem it not to be a use case
that the mainline kernel needs to be concerned about.

> Yes, but if the FIQ handler is also used for IPI, set_fiq_handler gets IPI
> interrupts (with the patch starting this thread)? So i think that the patch
> needs to look like:
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -483,6 +483,9 @@ asmlinkage void __exception_irq_entry
> handle_fiq_as_nmi(struct pt_regs *regs)
> +#ifndef CONFIG_FIQ
> gic_handle_fiq_ipi();
> #endif
> +#endif

No. With a single zImage kernel, you could very well have SMP and FIQ
both enabled, but have a non-SMP platform using FIQ, but also support
SMP platforms as well. Your change prevents that happening.

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