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

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Date:	Tue, 27 Oct 2015 14:02:48 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Leonid Yegoshin <Leonid.Yegoshin@...tec.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 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).

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.

David Daney

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