On 2/12/24 20:16, Johannes Berg wrote:
So ... I'm looking at the client side, and thinking about that.
According to the spec, multi-link element should be present in beacons
of APs affiliated with the same MLD if one of the (other) links is doing
CSA, and then have also the CSA counters of course, relative to the
target link's TBTT (of course.)
Yes correct, you are referring to critical update(s) right?
Theoretically, both mac80211 and hostapd today support different beacon
interval on different links, I believe.
This makes the whole thing of including CSA for link A on beacons/probe
responses transmitted on link B very tricky, because you have to know
the timing, etc.
At least handling for probe response seems to be a bit easier since
there we need not maintain any timer as such (as you have said too - we
need not be perfect there). But beacons, as you said, it is indeed bit
tricky to handle full support considering both links could beacon at
different intervals.
For the CSA counter of a link _itself_ we have counter_offsets_beacon
and counter_offsets_presp (for probe response offload) in struct
cfg80211_csa_settings, and also counter offsets in struct
cfg80211_mgmt_tx_params for sending probe responses.
But ... for the cross-link information in the MLE this gets way more
tricky? Especially if the beacon interval is different - we couldn't
just count down, we'd have to fill in the information when we know where
the frame is transmitted. For probe responses maybe we can afford to not
be perfect, but for beacons it better be right - so we have to calculate
the right counter value(s) for (all of) the switching link(s) according
to the current TSF, TSF offset and TBTT - not all of which we even have
in the driver?
Yes correct :)
I can see a few ways of implementing this:
a) Punt to firmware and it parses the multi-link element etc. to
find the places to update. But that's awful, it requires parsing
possibly fragmented MLE with fragmented subelements containing the
CSA elements inside ...
b) Punt to firmware and give it a (possibly long) list of offsets K_N
where to put the N'th counter for link K when transmitting the
frame.
At least for beacons if firmware takes care of it then it will be good.
Firmware could maintain the counter (for the affected link) and (I
assume it will be aware of the partner links) so whenever partner link
transmits a beacon it could add the CSA (or such IEs) in per STA profile
of the reporting link. It could get the contents from the affected link
but actual value of the counter could be filled from the global counter
being maintained. But not sure whether we can force every driver's
firmware to do so?
Let aside actual drivers, adding the test case for mac80211_hw_sim would
also be tricky since there beacon Tx is handled in software only.
c) Require the get_tsf operation and add an operation to retrieve TSF
offset (**), and then calculate the TBTT for each link when
beacon_get is called with the per-link beacon intervals and update
the values correctly if CSA is in progress on any link ... requires
against parsing like in (a) or offset information like in (b), but
at least now it's in software?
For probe responses this could be a bit off I guess, but maybe that
doesn't matter as much - probe responses are not authenticated so a
client probably shouldn't use them for real CSA anyway.
(**) which I guess we need anyway for hostapd to put it into beacons
etc.?
Yes I'm too anticipating hostapd changes to support the critical update.
d) Require beacon intervals to be the same, and then just count down?
Still requires the offset information etc., but that's also not
great because if the configuration happens during a TBTT on any of
the links (some before, some after), it'll be very wrong. So not
very inclined to do this one ...
Agreed. Can't enforce the intervals to be same.
Do you have any plans for any of this?
I do have some idea (possibly half baked code) for handling critical
updates in probe responses. Beacons, I assume firmware takes care hence
I have changes to mark it in the link_conf so that drivers could read
that and give it to its firmware.
Handling in beacon at kernel level, I need to think of it since anyways
hw_sim test case is good to have :)
Do we have any data right now, how many actual drivers have offloaded
beacon Tx to its firmware (at least those who are supporting Wi-Fi 7)?
If we could see majority are using firmware then we could first add
support for such drivers then we can think about handling it in kernel?
I'm mostly asking right now because I want a reliable way to test the
work I'm trying to do on the client side though, so I could also live
with some hacks (inject through debugfs?), but having it for real would
be nice.
Okay. I'm right now working to add basic test case in hw_sim for MLO CSA
(on first link) as we discussed. There is need of a few mac80211 changes
in order to support for other links. I'm checking those now. Once done
with these, then I'll take up critical updates.
