linux-kernel - Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock

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Date:	Tue, 27 Oct 2015 14:15:24 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	David Daney <ddaney.cavm@...il.com>
CC:	Ralf Baechle <ralf@...ux-mips.org>,
	Markos Chandras <markos.chandras@...tec.com>,
	<linux-mips@...ux-mips.org>, Alex Smith <alex.smith@...tec.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and
 clock_gettime()

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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