linux-kernel - Re: [PATCHv4 net-next] ptp/ioctl: support MONOTONIC_RAW timestamps for PTP_SYS_OFFSET

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Date: Tue, 07 May 2024 10:42:35 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Mahesh Bandewar <maheshb@...gle.com>, Netdev <netdev@...r.kernel.org>, 
 Linux <linux-kernel@...r.kernel.org>, David Miller <davem@...emloft.net>,
 Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Richard Cochran
 <richardcochran@...il.com>, Arnd Bergmann <arnd@...db.de>, Sagi Maimon
 <maimon.sagi@...il.com>
Cc: Jonathan Corbet <corbet@....net>, John Stultz <jstultz@...gle.com>, 
	Mahesh Bandewar <mahesh@...dewar.net>
Subject: Re: [PATCHv4 net-next] ptp/ioctl: support MONOTONIC_RAW timestamps
 for PTP_SYS_OFFSET_EXTENDED

On Thu, 2024-05-02 at 14:10 -0700, Mahesh Bandewar wrote:
> The ability to read the PHC (Physical Hardware Clock) alongside
> multiple system clocks is currently dependent on the specific
> hardware architecture. This limitation restricts the use of
> PTP_SYS_OFFSET_PRECISE to certain hardware configurations.
> 
> The generic soultion which would work across all architectures
> is to read the PHC along with the latency to perform PHC-read as
> offered by PTP_SYS_OFFSET_EXTENDED which provides pre and post
> timestamps.  However, these timestamps are currently limited
> to the CLOCK_REALTIME timebase. Since CLOCK_REALTIME is affected
> by NTP (or similar time synchronization services), it can
> experience significant jumps forward or backward. This hinders
> the precise latency measurements that PTP_SYS_OFFSET_EXTENDED
> is designed to provide.
> 
> This problem could be addressed by supporting MONOTONIC_RAW
> timestamps within PTP_SYS_OFFSET_EXTENDED. Unlike CLOCK_REALTIME
> or CLOCK_MONOTONIC, the MONOTONIC_RAW timebase is unaffected
> by NTP adjustments.
> 
> This enhancement can be implemented by utilizing one of the three
> reserved words within the PTP_SYS_OFFSET_EXTENDED struct to pass
> the clock-id for timestamps.  The current behavior aligns with
> clock-id for CLOCK_REALTIME timebase (value of 0), ensuring
> backward compatibility of the UAPI.
> 
> Signed-off-by: Mahesh Bandewar <maheshb@...gle.com>

LGTM, but let's wait a bit for Richard ack.

@Richard, could you please have look?

Paolo