On 2/13/24 14:12, Johannes Berg wrote:
On Tue, 2024-02-13 at 10:46 +0530, Aditya Kumar Singh wrote:
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?
I was actually thinking the critical update flag is rather not all that
interesting, but the inclusion of the CSA-related elements in a
(partial) per-STA profile in the Multi-Link Element is. Those contain
the countdown fields, so need to be updated - according to the TBTT of
the link doing the CSA, not the link sending it.
Yes correct, this CSA thing is interesting but there is one more thing -
handling of the BSS param change count (BPCC). Let's say the affected
link undergoes CSA, then in its Basic Multi Link IE, BPCC should be
incremented. But at the same time, in RNR of its partner links, where
this affected link is there, the BPCC should also be incremented.
All though here it is not incremented with every beacon Tx, but still
change in one affects beacon of other partner link(s) at least once.
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).
OTOH, if we already need some infrastructure for beacons, we can also
use that infrastructure to fill in probe responses? Might be easier
overall, because otherwise you'd have to start timers in hostapd etc. if
you actually wanted to fill that in hostapd, and that's messy ...
Correct!
And as a complete aside: link removal _also_ has a countdown, but I'm
not sure that affects cross-link, but if it does we might want a more
general cross-link counter infrastructure? Though that might not mesh
well with how (your) firmware handles it?
Yes.
But beacons, as you said, it is indeed bit
tricky to handle full support considering both links could beacon at
different intervals.
Indeed.
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.
Note that I'm basically asking you, Mediatek and Realtek how this should
work. We're almost certainly not going to support multi-link AP in our
device any time soon, and we could adjust the firmware to it anyway if
needed.
So while I agree that punting it to firmware would be somewhat nice, we
still have to agree on (a) or (b), because (b) requires more information
from hostapd etc., but (a) requires _substantially_ more (and very
complex) parsing in firmware... If you wanted to support multi-BSSID,
then it's even more complex, which would argue for (b)?
Correct. Both has got its own advantages and disadvantages. Need to
agree on any one so that all can be benefited. Let's hear from other folks.
I was trying to keep opinions out of this, but personally, I really
don't like (a), it puts too much complexity in the hands of firmware
engineers ;-)
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?
I'm not sure I understand that description - how would firmware even
know which elements to take from what partner link etc.? I think hostapd
still has to add the elements, and firmware would just populate the
counter field(s)? But yeah that requires knowing from which partner link
to populate.
Yeah so underlying assumption/thought was -
1. Firmware is aware of partner links
2. Host driver just needs to update the beacon template of the affected
link and some how indicate to firmware that this is critical update.
3. As per spec, there are around 21-22 changes classified as critical
update and among those only handful of those which has adding the IE in
per STA profile. So firmware could possibly maintain a table to see if
the affected link's beacon template has such IEs which if present needs
to be added in partner links, it could add it as and when partner link
transmits a beacon.
4. Once CSA is finished, the host would update the beacon template which
will not have CSA/ECSA IE... such IEs. Then firmware can stop adding to
per STA profile as well of the reporting links.
But yeah adding to per STA profile when already elements are there and
then first de-fragment and then add and then later fragment would be a
tedious job :)
If you really wanted the firmware to handle the partner link information
completely it'd not only have to parse the Multi-Link Element on other
links but also _modify_, and know what to do, etc. - I don't think
that'll really work so well.
Think about it some more: On the link doing the CSA you'd have to parse
the beacon template and understand where the CSA-related elements are
(CSA, ECSA, Channel Switch Wrapper, possibly more?), and copy them
aside. Then on the partner link(s) parse the Multi-Link Element, see if
there's a partial STA profile already for some reason, merge the CSA-
related elements into that, update the counters; all while first
removing the 2 levels of fragmentation, and then re-doing them ...
Yeah complex on firmware level. But even if we solve this on kernel
level, existing drivers who have offloaded beacon to firmware from 11ax
or before itself, still need to handle it? It would looks ugly that till
11ax it is offloaded and from 11be it is again put back to kernel?
I'm not even sure how I'd implement that in hwsim, and there I already
have all the helper functions and pretty much unlimited memory/code
size, so doing it in firmware seems awful?
Yes.
Not to mention that it would never extend for future stuff, since new
elements wouldn't be copied over, firmware wouldn't necessarily know the
right order to merge them with existing elements in the per-STA profile,
etc.
Yeah so if we go with handle on firmware level, as and when spec
changes, firmware changes are required too.
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.
Yeah but that's easy, especially if we go with (b) approach, even
mac80211 basically has all the necessary information to handle hwsim's
model of beacon transmission, we could add a little bit of additional
help there too (e.g. pass the TBTT to beacon_get).
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.
Oh hostapd will have to make changes here for everything - the (**)
footnote was about how hostapd knows the TSF offset between links?
Unless we're planning to always keep the TSF offset zero?
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 agree that we can go with one approach for now, but I think we should
agree on the first model of how firmware does this. I'm not convinced
that your description above where firmware even magically handles
propagating the CSA elements to partner links will really work? But see
above.
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.
