Re: [PATCHv4 net-next] ptp/ioctl: support MONOTONIC_RAW timestamps for PTP_SYS_OFFSET_EXTENDED

[Paolo Abeni <pabeni@redhat.com>]

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@google.com>

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

@Richard, could you please have look?

Paolo