MLO (Multi-Link Operation) support

[jetm]

Enable simultaneous multi-link WiFi 7 operation (2.4GHz + 5GHz STR, or 2.4GHz + 6GHz STR).

Background: MLO was included in v2.1-17 through v2.1-20 but dropped in v2.1-21 due to ROC timer issues and insufficient testing.

PR #22 re-adds MLO with targeted fixes:

• Force MLO_EN flag (firmware doesn't report capability in NIC_CAP)
• Fix MLO RX WCID lookup for BSS self-STA (def_wcid return)
• Skip 5GHz<->6GHz STR (shared RF1 radio on MT6639)

Test results (@Loong0x00, ASUS RT-BE92U, kernel 6.18.13):

Config	iperf3	PHY TX	Status
5GHz+2.4GHz STR	776 Mbps	2882 Mbps MCS13	Stable 30s
6GHz+2.4GHz STR	726 Mbps	310 Mbps MCS12	Stable 30s
3-link (6G+5G+2.4G)	625 Mbps	4322 Mbps MCS10	Stable 30s

MT6639 has 2 RFs: RF0=2.4GHz, RF1=5GHz/6GHz (shared). Only 2 simultaneous STR links possible.

Review findings (PR #22): The band exclusion in mac80211.c needs a chip guard - mt76_select_links() is shared with mt7996 which has 3 independent radios. See PR #22 review for details.

Plan: Separate follow-up series after the base 18-patch WiFi series lands upstream.

@marcin-fm could you test PR #22 on your TP-Link TBE550E setup? Interested in MLO stability and whether the 5GHz/6GHz band exclusion works correctly.
@Loong0x00 comprehensive testing looks great. For upstream, the 320MHz bss_rlm fix is being folded into the base series. The MLO-specific changes will be a separate follow-up.

Refs: PR #22, #8

[jetm]

@marcin-fm @Loong0x00 The base 18-patch WiFi series is ready for review:

• Kernel branch: https://github.com/jetm/linux-stable/tree/mt7927-wifi-support-v2

MLO is planned as a follow-up series after the base lands. Key blockers from the PR #22 review:

• mt76_select_links() band exclusion in mac80211.c breaks mt7996 (3 independent radios)
• wcid->def_wcid NULL in non-MLO paths needs a guard

@marcin-fm - if you have time to test MLO on your TP-Link Archer TBE550E + mt7996 AP setup, that would be very valuable for validating the follow-up series.

[jetm]

Sean Wang posted a 19-patch series to linux-wireless@ today that fixes MLO link STA lifetime and error handling in mt7925:

[PATCH 00/19] wifi: mt76: mt7925: fix up MLO link lifetime and error handling

The series fixes three categories of MLO bugs:

• Partially initialized links exposed via RCU before setup completes
• No host-side resource cleanup when firmware commands fail (MCU timeouts)
• Implicit mlink lookups in helpers creating hidden state dependencies

Key changes: explicit mlink passing to all helpers, RCU lifetime for link STA allocation, proper publish-after-success ordering, and host-side unwind on add failure.

This addresses the core stability issues that led us to defer MLO. Tracking for wireless-next landing.

[inverimus]

Is this currently enabled if I install from the AUR? I had to disable MLO on my router or wifi always stops working after a few minutes.

[jetm]

MLO is not included in the current DKMS package. The base driver has some MLO infrastructure from upstream mt7925, but three targeted fixes needed for stable operation have not been shipped yet.

If your router has MLO enabled, the client will attempt to negotiate it without the necessary driver support, causing disconnects. Disabling MLO on the router (as you did) is the correct workaround for now.

I'll update this issue when the MLO fixes are included in a release.