Porting airtime fairness (+ airtime queue limits?)

For Developers
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(This discussion will mainly circle on mwlwifi)

It's a sad state to see that some ath9k devices have more active development and longevity than some 802.11ac chips.

I want to attempt porting airtime fairness to my device that uses mwlwifi but I think I've already run into some roadblocks.

I know that mwlwifi is:

  1. Out-of-tree
  2. Dead, the Marvell Wireless business unit doesn't exist anymore (bought by NXP)
  3. There are barely any comments in the code
  4. A lot of stuff is locked behind the firmware

The code base is very similar to mwl8k.

I know that the following is needed for ATF support:

  • ieee80211_next_txq()
  • ieee80211_return_txq()
  • ieee80211_txq_schedule_start
  • ieee80211_txq_schedule_end

and I'll need to add the following attributes:

  • NL80211_STA_INFO_TX_DURATION
  • NL80211_STA_INFO_AIRTIME_WEIGHT
  • NL80211_EXT_FEATURE_AIRTIME_FAIRNESS

But both mwl8k and mwlwifi do not use ieee80211_txq. However, at least mwl8k has a starting point in mwl8k_txq_init.

I'm thinking I'll need to look more closely at one/some of these:

  • pcie_tx_init
  • pcie_tx_deinit
  • pcie_tx_skbs

Or some other stuff here: https://github.com/kaloz/mwlwifi/blob/master/hif/pcie/tx.c

I'd really like to know if there is something worth pursuing in this, or should I just give up...

The last time the code was reviewed by Linux wireless was: https://patchwork.kernel.org/patch/9482477/

Tagging the more knowledgeable people with Linksys WRT AC series hardware in this discussion (and have had longer ownership of this devices in this series than me):
@eduperez @mindwolf @solidus1983 @davidc502

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This does not directly answer your question, but the MT76 driver has AC and airtime fairness support. Might be an easier solution to switch to a MT76 device instead of trying to port the functionality. Especially since a much larger part of the driver is closed source for mlwifi in the form of firmware blobs. Thankfully the MT76 uses very small firmware blobs (although I would have preferred full opensource drivers of course)

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Would it be best to see what the blobs control first seems only logical to be honest.

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There is an active effort to mainline AQL at the mac80211 layer (see e.g. [PATCH v11 0/4] Add Airtime Queue Limits (AQL) to mac80211).

I don't know how this impacts mwlwifi/mwl8k and if this will add support for AQL in mwlwifi/mwl8k, but IIRC this mac80211 approach seeks to eliminate the need to add AQL from scratch in every single driver.

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It has already been pulled in by Linus for 5.5

It looks to be much easier to port over into mwl8k/mwlwifi.

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Backported the patch for OpenWrt Master-5f68333952 here. Copy as 700-Add-Airtime-Queue-Limits-Master-5f68333952.patch into {working_dir}/package/kernel/mac80211/subsys/

Have not compile-tested it yet.

EDIT: Missed a patch – https://pastebin.com/2EJnYznv

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Having just read through the code in a much more thorough manner, I don't think it will do anything differently for the Marvell Wireless chipsets. It still needs struct ieee80211_txq(). Neither mwlwifi nor mwl8k expose their txqs to mac80211. Thus, it won't benefit from this change.

Who will benefit? Current of users of ieee80211_txq() are [in Linux 5.4]:

  • ath10k
  • ath9k
  • iwlwifi
  • mt76

EDIT: I'm closing this since I've answered my own question – no it's not possible (at least with my level of C knowledge and driver coding) to port ATF and AQL for mwlwifi because it's queues aren't managed by mac80211. It will require drastic changes and a sizeable amount of time to refactor mwlwifi to use that.

I also had it wrong – one also needs to look into:
mwlwifi's mac80211.c file.
It contains things like:

  • mwl_get_rateinfo
  • mwl_mac80211_tx
  • mwl_mac80211_conf_tx
  • mwl_mac80211_get_stats
  • mwl_mac80211_ampdu_action

If we can somehow adopt the above to use ieee80211_txq, then the ATF and AQL features can follow.

EDIT 2:
Kindly pinging @dtaht and @tohojo for a second opinion on https://github.com/kaloz/mwlwifi/ (https://patchwork.kernel.org/patch/9482477/)

Thank you for taking the time in your busy days for this if you do read this.

4 Likes
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Will also check this out as well.

Edit: Just about to compile it now.

Edit 2: On a known good build it failed to recompile with that patch included however its compiled fine since after removing the patch.

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You're basically right that mwlwifi would need to be ported over to use the mac80211 TXQ path to make use of AQL and airtime fairness.

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And just a general note on terminology, since this can be confusing: What mac80211 calls a "TXQ" is an abstract queueing structure controlled by FQ-CoDel where packets are kept until they are ready to transmit. Sorta like a qdisc on the netdev, but adapted to WiFi usage.

Whereas the queues drivers manage are hardware queues, i.e., the pipelines of packets that the hardware/firmware manages. Some drivers (e.g., ath9k) use the name "txq" for these hardware queues as well, while others (e.g., mt76 I think), call them "hwq". So there is not necessarily a one-to-one relationship between a "mac80211 TXQ", and a "hardware queue". Although, to confuse matters further, I think that the iwl driver does actually couple mac80211 TXQs to hardware queues.

This is just to say that it's not just "porting a driver to use mac80211 txqs"; it's a different API between mac80211 and the driver entirely. Contrast figures 1 and 3 in this: https://www.usenix.org/conference/atc17/technical-sessions/presentation/hoilan-jorgesen

To get an idea about what it takes to port a driver, this is the patch that converted ath9k: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=50f08edf98096a68f01ff4566b605a25bf8e42ce

2 Likes
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I'd help port the driver but I have no idea how.
I've been trying to get some info from people related to this driver and either they don't answer or they don't know anything...

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