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Multi-Link Operation (MLO)

MT7925 supports Wi-Fi 7 Multi-Link Operation (MLO), which allows a device to maintain multiple simultaneous links to an access point for increased throughput and reliability.

Overview

Feature Value
Maximum Links 4 (IEEE80211_MLD_MAX_NUM_LINKS)
Default Link Link ID 0 (always present)
Additional Links Link IDs 1-3

Data Structures

Virtual Interface (VIF)

struct mt792x_vif {
    struct mt792x_bss_conf bss_conf;  // Default link (must be first)
    struct mt792x_bss_conf __rcu *link_conf[IEEE80211_MLD_MAX_NUM_LINKS];
    u16 valid_links;  // Bitmap of active links
    u8 deflink_id;    // Default link ID
};
  • bss_conf - Default link (always present)
  • link_conf[] - Additional links (RCU-protected)
  • valid_links - Bitmap indicating which links are active

Station (STA)

struct mt792x_sta {
    struct mt792x_link_sta deflink;  // Default link (must be first)
    struct mt792x_link_sta __rcu *link[IEEE80211_MLD_MAX_NUM_LINKS];
    u16 valid_links;  // Bitmap of active links
    u8 deflink_id;    // Default link ID
};
  • deflink - Default link state (always present)
  • link[] - Additional link states (RCU-protected)
  • Each link has its own WCID for hardware identification
struct mt792x_link_sta {
    struct mt76_wcid wcid;  // Hardware WCID (must be first)
    u32 airtime_ac[8];
    int ack_signal;
    struct mt792x_sta *sta;
};

struct mt792x_link_sta *mt792x_sta_to_link(struct mt792x_sta *msta, u8 link_id);

Can Return NULL

This function can return NULL during link transitions. Always check before dereferencing.

Get BSS Config

struct mt792x_bss_conf *mt792x_vif_to_link(struct mt792x_vif *mvif, u8 link_id);

Can Return NULL

This function can return NULL during link transitions. Always check before dereferencing.

Common MLO Patterns

for_each_set_bit(i, &valid_links, IEEE80211_MLD_MAX_NUM_LINKS) {
    struct ieee80211_bss_conf *link_conf;
    struct mt792x_bss_conf *mconf;

    link_conf = mt792x_vif_to_bss_conf(vif, i);
    if (!link_conf)  // CRITICAL: Check for NULL
        continue;

    mconf = mt792x_vif_to_link(mvif, i);
    // Use mconf
}

struct mt792x_link_sta *mlink = mt792x_sta_to_link(msta, link_id);
if (!mlink)  // CRITICAL: Check for NULL
    return -EINVAL;

wcid = &mlink->wcid;
// Use wcid

MLO Lifecycle

Entry: mt7925_change_vif_links()

  1. Allocate mt792x_bss_conf for new link
  2. Initialize link state
  3. Send MCU command to add link
  4. Update valid_links bitmap

Entry: mt7925_change_vif_links()

  1. Send MCU command to remove link
  2. Clear link from valid_links bitmap
  3. RCU-synchronize before freeing
  4. Free mt792x_bss_conf

During MLO Roaming:

  • Old link is torn down
  • New link is added
  • Transition period where links may be NULL

Critical

Always check for NULL during transitions to avoid kernel panics.

Common MLO Bugs

NULL Pointer Dereference

Bug Pattern: 

mlink = mt792x_sta_to_link(msta, link_id);
wcid = &mlink->wcid;  // Crash if mlink is NULL!

Fix: 

mlink = mt792x_sta_to_link(msta, link_id);
if (!mlink)
    return -EINVAL;
wcid = &mlink->wcid;

Missing Mutex Protection

Bug Pattern: 

for_each_set_bit(i, &valid_links, IEEE80211_MLD_MAX_NUM_LINKS) {
    bss_conf = mt792x_vif_to_bss_conf(vif, i);
    mt7925_mcu_uni_bss_ps(dev, bss_conf);  // MCU call without mutex!
}

Fix: 

mt792x_mutex_acquire(dev);
for_each_set_bit(i, &valid_links, IEEE80211_MLD_MAX_NUM_LINKS) {
    bss_conf = mt792x_vif_to_bss_conf(vif, i);
    if (!bss_conf)
        continue;
    mt7925_mcu_uni_bss_ps(dev, bss_conf);
}
mt792x_mutex_release(dev);

Bug Pattern: 

// Link is torn down
// mac80211 requests key removal
// link_conf is NULL -> returns -EINVAL

Fix (Patch 0012): 

if (!link_conf || !mconf || !mlink) {
    // During MLO roaming, link may be torn down before key removal
    if (cmd != SET_KEY)
        return 0;  // Successfully removed (link already gone)
    return -EINVAL;
}

6GHz Considerations

6GHz band with MLO can trigger issues:

  • Regulatory domain switching
  • Channel availability differences between bands
  • Firmware state machine issues during band transitions

Workarounds

If you experience frequent issues with 6GHz MLO:

# Check current regulatory domain
iw reg get

# Set regulatory domain explicitly
sudo iw reg set US

Consider using 2.4GHz + 5GHz only if 6GHz causes problems.

Testing MLO

Requirements

  • Wi-Fi 7 capable AP with MLO enabled
  • MLO-enabled firmware on MT7925
  • Multiple bands/channels available (2.4GHz, 5GHz, 6GHz)

Verify MLO

# Check link count
iw dev wlan0 link
# Should show multiple links

# Check kernel logs
dmesg | grep -i "link\|mlo"
# Should show link addition/removal
Patch Description
0005 NULL checks for MLO operations
0009 Fix MLO roaming and ROC setup
0012 Fix key removal during MLO roaming