Re: [PATCH 3.17-rc4 v7 0/6] arm: Implement arch_trigger_all_cpu_backtrace

[Russell King - ARM Linux]

On Thu, Oct 16, 2014 at 10:23:52AM +0100, Daniel Thompson wrote:
> On 14/10/14 23:37, Daniel Drake wrote:
> > I'm testing your patches on Exynos4412 and I guess in their current
> > state they don't go quite this deep, as the only callers of
> > trigger_all_cpu_backtrace() are sysrq, hung_task and spinlock debug
> > code - none of which seem as fail-safe as a trigger like a
> > pre-programmed watchdog NMI interrupt would be.
> > 
> > Do I need to find a way to get CONFIG_FIQ available on this platform
> > first? and/or CONFIG_HARDLOCKUP_DETECTOR?
> 
> You need CONFIG_FIQ working first. Be aware that this may be impossible
> on Exynos unless you control the TrustZone. For this reason most of my
> development is on Freescale i.MX6 (because i.MX6 boots in secure mode).

CONFIG_FIQ enables the legacy FIQ code which is unsuitable for use on
SMP, so that should not be a requirement.

We still need to validate all the code we're proposing to run in FIQ
context does not violate any locking.  IRQ-safe locks will do not
prevent FIQs occuring, and using IRQ-safe locks which are also taken
in the FIQ path /will/ cause deadlocks.  So, we need to ensure that
the perf internals are safe for this.

Lastly, platforms running in non-secure mode most likely will not be
able to take /any/ advantage from the FIQ stuff because FIQs will 
likely only be available to the secure firmware.

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