Re: [RFC PATCH v2 0/8] timekeeping: Fix draft tracking precision and add feed-forward discipline via vmclock

[Thomas Gleixner <tglx@kernel.org>]

On Fri, May 22 2026 at 17:50, David Woodhouse wrote:
> On Fri, 2026-05-22 at 17:28 +0200, Thomas Gleixner wrote:
>> 
>>   git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git timers/ptp/timekeeping
>
> In 94dd85a8d0a ("timekeeping: Add system_counterval_t to struct
> system_device_crosststamp") my version ditched the system_counterval_t
> on the stack and just used the one in xtstamp directly.

Which is wrong. I did it the way I did for a very good reason.

> The convert_base_to_cs() function probably wants to scv->id=cs->id for
> itself anyway; otherwise it's leaving behind an inconsistent
> system_counterval_t object which... will lead to exactly the bug my
> first version of that had, that I see you avoided :)

No. It can't because that would corrupt the object for the retry case,
which would then hand back the wrong value.

The object _IS_ consistent because the csid in there is related to the
PTM value and not to the clocksource. The function updates the @cycles
value and leaves everything else untouched. The clock ID for the @cyles
value is guaranteed to be the clock ID of the system clocksource, so
using this is the right thing to do.

Just because it looks tempting or your AI buddy told you so doesn't make
it correct.

Thanks,

        tglx