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From:   "Stern, Avraham" <avraham.stern@intel.com>
To:     Richard Cochran <richardcochran@gmail.com>,
        "Greenman, Gregory" <gregory.greenman@intel.com>
CC:     "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
Thread-Topic: pull-request: wireless-next-2023-03-30
Thread-Index: AQHZbbjNzP9gzb4bwEuw1qGhSN1B8K8xGe8AgAJ/PgCABVt22w==
Date:   Sun, 23 Apr 2023 13:33:19 +0000
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hi Richard,

I will try to clarify.


> > Then, the timestamps are added to the rx/tx status
>> via mac80211 api.

> Where?  I don't see that in the kernel anywhere.

> Your WiFi driver would need to implement get_ts_info, no?

so, the rx/tx timestamp is put in the skb hwstamps field, and the ack (tx/r=
x) timestamp is put in the mac80211 rx/tx status.
if you follow mac80211/cfg80211 patches sent earlier, you'll see that mac80=
211 uses these to fill cfg80211_rx_info and cfg80211_tx_status with
the timestamps.
eventually, these are sent to userspace in nl80211_send_mgmt() and nl80211_=
frame_tx_status() as part of the frame's meta data.

since wifi uses management frames for time sync, the timestamping capabilit=
y is also advertised using nl80211 capability (NL80211_ATTR_MAX_HW_TIMESTAM=
P_PEERS).
implementing get_ts_info() doesn't seem right since it's usually queried ov=
er a data socket, and the wifi driver doesn't timestamp data frames (since =
these are not used
for time sync over wifi).

>> Actually, we already have a functional implementation of ptp4l
>> over wifi using this driver support.

> Why are changes needed to user space at all?

As you mentioned, time sync over wifi leverages the existing FTM protocol, =
which is different from the protocols used over ethernet.
In particular, FTM uses management frames unlike the ethernet protocols tha=
t use data frames.
So obviously for ptp4l to support time sync over wifi, it will need to impl=
ement the FTM protocol (sending FTM frames via nl80211 socket) and use the =
kernel APIs added here
to get tx/rx and acks timestamps. Then it can pass the collected data (t1, =
t2, path delay etc.) to the existing logic to calculate the time sync itsel=
f.
Note that for time sync the FTM frames also need to contain a vendor specif=
ic IE with all the time sync information (which is dynamic since it contain=
s timestamps data from each measurement), so using FTM offload is not a goo=
d option.

I hope this clarifies thing a bit.

Thanks,
Avi

________________________________________
From: Richard Cochran <richardcochran@gmail.com>
Sent: Thursday, April 20, 2023 6:43 AM
To: Greenman, Gregory
Cc: kuba@kernel.org; linux-wireless@vger.kernel.org; johannes@sipsolutions.=
net; netdev@vger.kernel.org; Stern, Avraham
Subject: Re: pull-request: wireless-next-2023-03-30

On Tue, Apr 18, 2023 at 01:35:50PM +0000, Greenman, Gregory wrote:

> Just a few clarifications. These two notifications are internal to iwlwif=
i, sent
> by the firmware to the driver.

Obviously.

> Then, the timestamps are added to the rx/tx status
> via mac80211 api.

Where?  I don't see that in the kernel anywhere.

Your WiFi driver would need to implement get_ts_info, no?

> Actually, we already have a functional implementation of ptp4l
> over wifi using this driver support.

Why are changes needed to user space at all?

Thanks,
Richard
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