RFC clocksource: exynos_mct: allow mct to use 64-bit counter from coprocessor

Hi all,

year(s) ago it was discovered that MCT timer and ARM architectured timer
are the same hardware with different interface. Here [1].

I followed mail-list discussions about removing MCT and using arch timer for Exynos5-based SoCs but things aren't moving at least latest upstream kernel on odroid-xu3 will use MCT as default timers. Maybe the reason are some power-management related things that very specific to Samsung. I don't know.

Idea of this draft patchset comes from Doug patches when he tried to optimize read of 64-bit counter located in mmio. [2]
Why not using cp15 counter instead if possible?

Previous numbers for 1000000 gettimeofday() calls from userspace are about 1 ms. With this patches we have 0.5 ms or even better. So twice as fast.

Just as matter of interest i tried to perform 2000000 sched_yield() calls from userspace. I see around 20% speedup with patches applied.

I tried to use hackbench but it's hard to feel the difference, maybe speedup is 0.5% but with bad fluctuation.

Everything is tested on odroid-xu3.
Looks like Cortex-A9 based Samsung SoCs (odroid-u2, odroid-x) don't have arch timer.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014
-May/256943.html
[2] https://lkml.org/lkml/2014/6/20/431

Current code created with such assumptions:
-- don't want to insert huge refactoring in MCT code;
-- target platform only ARMv7 Exynos5-based SoC that works in 32-bit mode so i don't want to build described functionality on ARM64. Currently i'm trying to patch odroid-xu3 DT only.
-- firmware on odroid-xu3 is broken and secondary cores start in SVC mode instead of HYP mode. Maybe i can remove check for hyp mode; -- in addition instead of DT property I may parse PFR regs and find Generic Timer Extension there.

I hope you are not getting bored reading to me. Current code is a little bit draft so comments and ideas are welcome.

Best regards,
Alexey Klimov