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Date:   Tue, 25 Apr 2023 19:43:40 -0700
From:   Richard Cochran <richardcochran@gmail.com>
To:     "Stern, Avraham" <avraham.stern@intel.com>
Cc:     "Greenman, Gregory" <gregory.greenman@intel.com>,
        "kuba@kernel.org" <kuba@kernel.org>,
        "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        "johannes@sipsolutions.net" <johannes@sipsolutions.net>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: pull-request: wireless-next-2023-03-30
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On Tue, Apr 25, 2023 at 07:03:50AM +0000, Stern, Avraham wrote:

> Having the timestamps of the frames seemed like a basic capability that userspace will need to implement ptp over wifi, regardless of the selected approach.

Having time stamps on unicast PTP frames would be a great solution.
But I'm guessing that you aren't talking about that?

> Apparently you had other ways in mind, so I would love to have that discussion and hear about it.  

Let's back up a bit.  Since you would like to implement PTP over Wifi
in Linux, may I suggest that the first step is to write up and
publish your design idea so that everyone gets on the same page?

Your design might touch upon a number of points...

- Background
  - Difficulty of multicast protocols (like PTP) over WiFi.
  - What do the networking standards say?
    - IEEE Std 802.11-2016
      - Timing Measurement (TM)
      - Fine Timing Measurement (FTM)
    - IEEE 1588
      - Media-Dependent, Media-Independent MDMI
      - Special Ports
    - 802.1AS
      - Fine Timing Measurement Burst
  - Which of the above can be used for a practical solution?
    - What are the advantages/disadvantages of TM versus FTM?
    - What alternatives might we pursue?
      - unicast PTP without FTM
      - AP as transparent clock
- Existing Linux interfaces for time synchronization
  - What can be used as is?
  - What new interaces or extensions are needed, and why?
- Vendor support
  - How will we encourage broad acceptance/coverage?

IMO, the simplest way that will unlock many use cases is to provide
time stamps for single unicast frames, like in
ieee80211_rx_status.device_timestamp and expose an adjustable PHC
using timecounter/cyclecounter over the free running usec clock.  Then
you could synchronize client/AP over unicast IPv4 PTP (for example)
with no user space changes needed, AND it would work on all radios,
even those that don't implement FTM.

Thanks,
Richard