From: Arnd Bergmann <arnd@arndb.de>
To: y2038@lists.linaro.org
Cc: Ed Cashin <ed.cashin@acm.org>, Tina Ruchandani <ruchandani.tina@gmail.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [Y2038] [PATCH] aoe: Use 64-bit timestamp in frame
Date: Tue, 12 May 2015 13:14:10 +0200
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On Tuesday 12 May 2015 11:44:21 Arnd Bergmann wrote:
> 
> There are of course multiple ways to do this. One way would be to
> change the code to work on 32-bit nanoseconds instead of 32-bit
> microseconds. This requires proving that the we cannot exceed
> 4.29 seconds of round-trip time in calc_rttavg().
> Is that a valid assumption or not?
> 
> If not, we could replace do_gettimeofday() with ktime_get_ts64().
> This will ensure we don't need a 64-bit division when converting
> the ts64 to a 32-bit microsecond value, and combined with the
> conversion is still no slower than do_gettimeofday(), and it
> still avoids the double bookkeeping because it uses a monotonic
> timebase that is robust against settimeofday.

Two other approaches that occurred to me later:

- introduce common ktime_get_ms(), ktime_get_us(), ktime_get_real_ms()
  and ktime_get_real_is() interfaces, to match the other interfaces
  we already provide. These could be done as efficiently or better
  than what aoe does manually today.

- change the timebase that is used for the computations in aoe to use
  scaled nanoseconds instead of microseconds. Using 

  u32 time = ktime_get_ns() >> 10;

  would give you a similar range and precision as microseconds, but
  completely avoid integer division. You could also use a different
  shift value to either extend the range beyond 71 minutes, or the
  extend the precision to something below a microsecond. This would
  be the most efficient implementation, but also require significant
  changes to the driver.

	Arnd
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