linux-kernel - Re: [PATCH] powerpc: Convert VDSO update function to use new update

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Date:   Wed, 21 Jun 2017 11:51:42 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Paul Mackerras <paulus@...abs.org>
Cc:     linuxppc-dev@...abs.org, Michael Ellerman <michael@...erman.id.au>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc: Convert VDSO update function to use new
 update_vsyscall interface

On Sat, May 27, 2017 at 1:04 AM, Paul Mackerras <paulus@...abs.org> wrote:
> This converts the powerpc VDSO time update function to use the new
> interface introduced in commit 576094b7f0aa ("time: Introduce new
> GENERIC_TIME_VSYSCALL", 2012-09-11).  Where the old interface gave
> us the time as of the last update in seconds and whole nanoseconds,
> with the new interface we get the nanoseconds part effectively in
> a binary fixed-point format with tk->tkr_mono.shift bits to the
> right of the binary point.
>
> With the old interface, the fractional nanoseconds got truncated,
> meaning that the value returned by the VDSO clock_gettime function
> would have about 1ns of jitter in it compared to the value computed
> by the generic timekeeping code in the kernel.
>
> The powerpc VDSO time functions (clock_gettime and gettimeofday)
> already work in units of 2^-32 seconds, or 0.23283 ns, because that
> makes it simple to split the result into seconds and fractional
> seconds, and represent the fractional seconds in either microseconds
> or nanoseconds.  This is good enough accuracy for now, so this patch
> avoids changing how the VDSO works or the interface in the VDSO data
> page.
>
> This patch converts the powerpc update_vsyscall_old to be called
> update_vsyscall and use the new interface.  We convert the fractional
> second to units of 2^-32 seconds without truncating to whole nanoseconds.
> (There is still a conversion to whole nanoseconds for any legacy users
> of the vdso_data/systemcfg stamp_xtime field.)
>
> In addition, this improves the accuracy of the computation of tb_to_xs
> for those systems with high-frequency timebase clocks (>= 268.5 MHz)
> by doing the right shift in two parts, one before the multiplication and
> one after, rather than doing the right shift before the multiplication.
> (We can't do all of the right shift after the multiplication unless we
> use 128-bit arithmetic.)
>
> Signed-off-by: Paul Mackerras <paulus@...abs.org>

Apologies again for missing this earlier.

So no objections from me. I can't say I really worked the whole thing
out, but you're handling the xtime_nsec field properly and the rest
looks reasonable and is well documented.

So for what its worth:
Acked-by: John Stultz <john.stultz@...aro.org>

Thanks again for making this update!
-john