Re: [PATCH] ia64: fix clock_getre(CLOCK_MONOTONIC) to report ITC frequency

matoro <matoro_mailinglist_kernel@xxxxxxxxx> · Wed, 17 Aug 2022 17:57:42 -0400

Hi Andrew, I came up with the prototype for this patch, but it was based 

entirely on Sergei's investigation which was documented in 

https://bugs.gentoo.org/596382.  I asked him to send it upstream because 

I'm unable to attach my realname to it due to my job.  I can place a 

signed-off-by with my handle but I understand that's normally against 

kernel policy which is why I didn't.  Either way the bulk of the work 

belongs to Sergei, I just scribbled it down, and he cleaned it up for 

this submission.

-------- Original Message --------

Subject: Re: [PATCH] ia64: fix clock_getre(CLOCK_MONOTONIC) to report 

ITC frequency

Date: 2022-08-17 15:21
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
To: Sergei Trofimovich <slyich@xxxxxxxxx>

On Mon, 15 Aug 2022 06:49:44 +0100 Sergei Trofimovich <slyich@xxxxxxxxx> 

wrote:

clock_gettime(CLOCK_MONOTONIC, &tp) is very precise on ia64 as it uses
ITC (similar to rdtsc on x86). It's not quite a hrtimer as it is a few
times slower than 1ns. Usually 2-3ns.

clock_getres(CLOCK_MONOTONIC, &res) never reflected that fact and
reported 0.04s precision (1/HZ value).

In https://bugs.gentoo.org/596382 gstreamer's test suite failed loudly
when it noticed precision discrepancy.

Before the change:

    clock_getres(CLOCK_MONOTONIC, &res) reported 250Hz precision.

After the change:

    clock_getres(CLOCK_MONOTONIC, &res) reports ITC (400Mhz) precision.

The patch is based on matoro's fix. It adds a bit of explanation why we
need to special-case arch-specific clock_getres().

It would be best (and nice) to include the original developer's
Signed-off-by: and to Cc Émeric Maschino if possible?

Could you please take care of these paperwork issues?