Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime()

From	Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Newsgroups	linux.kernel
Subject	Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime()
Date	2015-10-27 22:20 +0100
Message-ID	<qojOX-1Qn-53@gated-at.bofh.it> (permalink)
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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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Thread

[PATCH v3 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime() Markos Chandras <markos.chandras@imgtec.com> - 2015-10-21 11:00 +0200
  Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Leonid Yegoshin <Leonid.Yegoshin@imgtec.com> - 2015-10-23 03:50 +0200
    Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Ralf Baechle <ralf@linux-mips.org> - 2015-10-27 15:50 +0100
      Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Leonid Yegoshin <Leonid.Yegoshin@imgtec.com> - 2015-10-27 21:50 +0100
        Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() David Daney <ddaney.cavm@gmail.com> - 2015-10-27 22:10 +0100
          Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Leonid Yegoshin <Leonid.Yegoshin@imgtec.com> - 2015-10-27 22:20 +0100
            Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Leonid Yegoshin <Leonid.Yegoshin@imgtec.com> - 2015-10-27 22:50 +0100
              Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime() Alex Smith <alex@alex-smith.me.uk> - 2015-10-28 11:30 +0100
                Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Leonid Yegoshin <Leonid.Yegoshin@imgtec.com> - 2015-10-28 19:30 +0100
                Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime() Alex Smith <alex@alex-smith.me.uk> - 2015-10-28 19:40 +0100
                Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Leonid Yegoshin <Leonid.Yegoshin@imgtec.com> - 2015-10-28 20:00 +0100
                Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime() Alex Smith <alex@alex-smith.me.uk> - 2015-10-28 20:20 +0100
                Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Leonid Yegoshin <Leonid.Yegoshin@imgtec.com> - 2015-10-28 20:30 +0100
                Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime() Alex Smith <alex@alex-smith.me.uk> - 2015-10-28 21:00 +0100
                Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() Leonid Yegoshin <Leonid.Yegoshin@imgtec.com> - 2015-10-28 21:20 +0100
            Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and  clock_gettime() David Daney <ddaney.cavm@gmail.com> - 2015-10-27 22:50 +0100

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