Messages in this thread | | | Date | Tue, 27 Oct 2015 14:15:24 -0700 | From | Leonid Yegoshin <> | Subject | Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime() |
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On 10/27/2015 02:02 PM, David Daney wrote: > On 10/27/2015 01:46 PM, Leonid Yegoshin wrote: > [...] >> >> And finally. clock scaling - what we would do if there are two CPUs with >> different clock ratios in system? It seems like common kernel timing >> subsystem can handle that. >> > > The code that executes in userspace must have access to a consistent > clock source. If you are running on a SMP system that doesn't have > synchronized CP0.Count registers, then your gettimeofday() cannot use > CP0.Count (RDHWR $2).
Right, I agree.
> > As far as I know, CP0.Count is the only available counter visible to > userspace, so you would have to disable the accelerated versions of > gettimeofday() where you cannot assert that the counters are always > synchronized.
Any system with GIC may have access to the same GIC global counter in a special separate page available for mapping by user in RO mode and it seems Alex did that.
Besides that this GIC global counter is used as a major system clocksource in systems with GIC.
- Leonid
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