ARM: vdso gettimeofday using generic timer architecture

Quoted 19 lines · Stephen Boyd

Quoted 18 lines · Nathan Lynch

Provide fast userspace implementations of gettimeofday and clock_gettime on systems that implement the generic timers extension defined in ARMv7. This follows the example of arm64 in conception but significantly differs in some aspects of the implementation (C vs assembly, mainly).

Clocks supported:
- CLOCK_REALTIME
- CLOCK_MONOTONIC
- CLOCK_REALTIME_COARSE
- CLOCK_MONOTONIC_COARSE

This also provides clock_getres (as arm64 does).

Note that while the high-precision realtime and monotonic clock support depends on the generic timers extension, support for clock_getres and coarse clocks is independent of the timer implementation and is provided unconditionally.

Quoted 2 lines

I think we'll need to rename the clocksource in arch_timer.c if we only have an mmio architected timer to something like arch_mem_counter.

I guess ARMv7 would allow you to have mmio without cp15 (AFAIK ARMv8 does not). I don't see any dts in arch/arm that contains an arm,armv7-timer-mem node without an arm,armv7-timer node, though.

If this is a practical concern, I think using CONFIG_ARCH_CLOCKSOURCE_DATA is perhaps a better way to communicate whether cp15 access is available. That's how the x86 vdso, for example, decides between using HPET vs TSC etc.