linux-kernel - Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross timestamps infrastructure

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Message-ID: <20170123184415.GA2158@localhost.localdomain>
Date:   Mon, 23 Jan 2017 19:44:15 +0100
From:   Richard Cochran <richardcochran@...il.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross
 timestamps infrastructure

On Mon, Jan 23, 2017 at 11:19:17AM -0200, Marcelo Tosatti wrote:
> This is needed to generate the PTP_SYS_OFFSET data: a table with read
> from realtime clock, read from device clock, read from realtime clock,
> ... :
> 
> time ->
> device clock	|	|sample2|	|sample4|	|sample6| ...
> -------------------------------------------------------------
> realtime clock  |sample1|       |sample3|	|sample5|

Here "realtime clock" is CLOCK_REALTIME on the guest, and "device
clock" is CLOCK_REALTIME on the host?
 
> Where sampleN is the read of the respective clock.
> 
> From the following PTP_SYS_OFFSET_PRECISE data:
> 
> device clock	|sample1P,deviceclock|       |sample2P,deviceclock|
> -------------------------------------------------------------
> realtime clock  |sample1P,realtimeclock|     |sample2P,realtimeclock|

Are |sample1P,deviceclock| and |sample1P,realtimeclock| taken at the
same instant in time?

If not, then calling that PTP_SYS_OFFSET_PRECISE is misleading.

Still don't get what you are doing here...

Thanks,
Richard