linux-kernel - Re: [RFC v2] ARM: sched_clock: update epoch

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Date:	Thu, 2 Aug 2012 18:28:47 -0700
From:	Colin Cross <ccross@...roid.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Barry Song <21cnbao@...il.com>,
	Vaibhav Bedia <vaibhav.bedia@...com>,
	Krzysztof Halasa <khc@...waw.pl>,
	Nicolas Pitre <nico@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC v2] ARM: sched_clock: update epoch_cyc on resume

On Fri, Jul 27, 2012 at 8:30 PM, Colin Cross <ccross@...roid.com> wrote:
> On Fri, Jul 27, 2012 at 6:15 PM, Colin Cross <ccross@...roid.com> wrote:
>> That patch was merged in 3.4, and my patch is on top of it.  Your
>> patch updates epoch_cyc and epoch_ns in suspend, but if the first call
>> to cyc_to_sched_clock after resume gets cyc = 0, cyc - epoch_cyc can
>> be negative, although it will be cast back to a large positive number.
>>
>> With my patch, epoch_cyc is updated in resume to whatever
>> read_sched_clock() returns, and epoch_ns is still set to the suspend
>> value, so it appears that sched_clock has not changed between
>> sched_clock_suspend and sched_clock_resume.  It will work with any
>> timer behavior (reset to 0, reset to random value, or continuing
>> counting).  The old setup_sched_clock function maintains the old
>> behavior to appease those who like their 32kHz sched clock to continue
>> ticking in suspend, although I think it is more correct for all sched
>> clocks to be faster than 32 kHz (when possible) and stop in suspend.
>
> I think the existing code can cause sched_clock to go backwards if the
> register read by the read function resets to 0 in suspend:
>
> In sched_clock_suspend epoch_cyc is updated to 0x00002000.
>
> The first sched_clock call after resume gets cyc = 0, returning
> epoch_ns + cyc_to_ns(0xffffe000)
>
> Later, sched_clock gets cyc = 0x5000, returning epoch_ns +
> cyc_to_ns(0x3000), which will be much smaller than the previous
> sched_clock value.

Russell, any update on this?  Should sched_clock.c handle read
functions that go backwards in suspend, or should I modify the read
function to track an offset in suspend and always return a
monotonically increasing value across suspend?
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