Implement dynamic MCS selection based on signal strength in vWIFI driver #80

dingsen-Greenhorn · 2025-06-07T20:19:57Z

This commit enhances the vWIFI driver by implementing dynamic Modulation and Coding Scheme (MCS) selection in the vwifi_get_station function, adjusting the MCS index based on signal strength

After implement dynamic MCS can avoid TX power waste for a bad channel quality

jserv

Follow the consistent coding style.

jychen0611

It seems that you are adjusting the MCS based on the current signal strength. However, what if a user wants to set the MCS manually using a command like iw dev wlan0 set bitrates ht-mcs-2.4 <mcs_index>?

To support this, you could consider implementing the set_bitrate_mask() callback in your cfg80211_ops and storing the specified MCS in the corresponding vwifi interface structure.

dingsen-Greenhorn · 2025-06-08T06:43:49Z

It seems that you are adjusting the MCS based on the current signal strength. However, what if a user wants to set the MCS manually using a command like iw dev wlan0 set bitrates ht-mcs-2.4 <mcs_index>?

To support this, you could consider implementing the set_bitrate_mask() callback in your cfg80211_ops and storing the specified MCS in the corresponding vwifi interface structure.

Thanks for you suggestion ,I can try to included the MCS for maunally adjust !

jychen0611

Can you explain in more detail how your design handles MCS settings? In particular, the MCS formula.

Also, why is the MCS index constrained to 7, 15, 23 and 31? Please clarify the reasoning behind these limits.

jychen0611 · 2025-06-08T15:41:09Z

vwifi.c

@@ -88,6 +88,8 @@ struct vwifi_vif {
    struct wireless_dev wdev;
    struct net_device *ndev;
    struct net_device_stats stats;
+    int manual_mcs;      /* ADDED: Store user-specified MCS */


Remove "ADDED:" prefix from the comment

jychen0611 · 2025-06-08T15:41:46Z

vwifi.c

@@ -88,6 +88,8 @@ struct vwifi_vif {
    struct wireless_dev wdev;
    struct net_device *ndev;
    struct net_device_stats stats;
+    int manual_mcs;      /* ADDED: Store user-specified MCS */
+    bool manual_mcs_set; /* ADDED: Flag to indicate manual MCS override */


Remove "ADDED:" prefix from the comment

jychen0611 · 2025-06-08T15:45:38Z

vwifi.c

@@ -1728,13 +1743,8 @@ static int vwifi_start_ap(struct wiphy *wiphy,

    /* Initialize hrtimer of beacon */
    pr_info("vwifi: init beacon_timer.\n");
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 15, 0)


Don't modify unrelated code segment!
You should align with the latest version of vwifi.

dingsen-Greenhorn · 2025-06-08T17:02:23Z

Can you explain in more detail how your design handles MCS settings? In particular, the MCS formula.

Also, why is the MCS index constrained to 7, 15, 23 and 31? Please clarify the reasoning behind these limits.

First,the code handles MCS selection and modulation assignment, either manually (based on vif->manual_mcs_set and vif->manual_mcs) or automatically (based on signal strength).

Second,The vWIFI driver constrains MCS indices to 7, 15, 23, and 31 to simplify rate calculation, testing, and implementation. This avoids the need for a comprehensive calculation of data-rates to handle all 32 MCS indices, varying streams, and channel widths. The chosen indices represent a range of modulations (BPSK to 64-QAM) for 4 streams, supporting both manual (iw commands) and automatic (signal-based) modes. The code’s design prioritizes simplicity for a virtual driver, ensuring functionality (as shown in your test output) while limiting complexity.

And thanks for you thoughtful question,it might be the next PR to optimize it !

EricccTaiwan · 2025-06-08T21:17:40Z

Nit: It might be better to squash all the “Fix coding style” commits.

jychen0611 · 2025-06-10T06:40:09Z

scripts/hostapd.conf

@@ -2,7 +2,7 @@ interface=vw0
 driver=nl80211
 debug=1
 ctrl_interface=/var/run/hostapd
-ctrl_interface_group=0
+ctrl_interface_group=root


The changes to the MCS configuration logic and the hostapd.conf control group setting serve different purposes and should be separated into distinct PRs.

The MCS logic affects runtime behavior of the vwifi driver, while the ctrl_interface_group=root change only impacts permission control for the hostapd socket.

Splitting them would improve clarity, ease future debugging, and allow better tracking of changes.

jychen0611 · 2025-06-10T07:27:12Z

vwifi.c

+    const char *modulation;
+    if (vif->manual_mcs_set) {
+        mcs_index = vif->manual_mcs;
+        switch (mcs_index) {


According to IEEE 802.11n, MCS indices 7, 15, 23, and 31 all use 64-QAM modulation.
You should revise the descriptions associated with each MCS index accordingly, or consider modifying your design if your intention was to differentiate based on modulation type.

ref:
mcsindex

dingsen-Greenhorn · 2025-06-12T16:23:25Z

first is auto selection MCS ,and manual assign three difference MCS for test.

jychen0611

Why is there a shell script embedded in the kernel module???
Use command

$ clang-format -i *.[ch]

to maintain a consistent coding style in your implementation!

jychen0611 · 2025-06-12T16:24:54Z

vwifi.c

-    {                              \
-        .bitrate = (_rate),        \
-        .hw_value = (_hw_value),   \
+for file in ${SOURCES}


Why is there a shell script embedded in the kernel module???
Use command

$ clang-format -i *.[ch]

to maintain a consistent coding style in your implementation!

Update hrtimer initialization to use the new hrtimer_setup() interface introduced in Linux commit 908a1d7 (hrtimers: Introduce hrtimer_setup() to replace hrtimer_init()),which replaces hrtimer_init() with a type-safe and simplified API. The new interface requires a hrtimer_callback function pointer to be explicitly provided at initialization. This commit identifies and passes the appropriate callback from the existing code to maintain equivalent behavior. The legacy hrtimer_init() remains available but is discouraged in new code. Compatibility is preserved using version checks against KERNEL_VERSION(6, 15, 0).

This commit enhances the vWIFI driver by implementing dynamic Modulation and Coding Scheme (MCS) selection in the `vwifi_get_station` function, adjusting the MCS index based on signal strength. After implement dynamic MCS can avoid TX power waste for a bad channel quality.

Implement the set_bitrate_mask callback in cfg80211_ops to support manual MCS settings using `iw dev <interface> set bitrates ht-mcs-2.4 <mcs_index>`, addressing reviewer feedback. Store the selected MCS in vwifi_vif->manual_mcs and track its state with vwifi_vif->manual_mcs_set. Support MCS indices 7, 15, 23, and 31, with validation and logging. Enable High Throughput (HT) in nf_band_2ghz with MCS 0–31, using IEEE80211_HT_CAP_SUP_WIDTH_20_40 for 20 MHz compatibility. Fix compilation errors by initializing rx_mask statically and removing const qualifiers from channel/rate arrays. Improve vwifi_connect to ensure stable association. Tested with `iw dev vw1 set bitrates ht-mcs-2.4 15`, achieving MCS 15 at 130.0 MBit/s (bitrate calculation pending refinement to ~52 MBit/s). Test commands format $sudo ip netns exec ns1 iw dev vw1 link $sudo ip netns exec ns1 iw dev vw1 set bitrates ht-mcs-2.4 15 /*(changable 7,15,23,31)*/ $sudo ip netns exec ns1 iw dev vw1 link

Update the vwifi_set_bitrate_mask callback to support the full range of MCS indices 24 through 31, aligning with the 4-spatial-stream configuration in nf_band_2ghz. This change enabling support for manual MCS from 24-31 coding schemes (1/2, 3/4, 2/3, 5/6) and modulations (BPSK, QPSK,16-QAM, 64-QAM) as defined by the IEEE 802.11n specification. The callback now rejects indices outside 24–31, ensuring compliance with 4-stream capabilities and allowing comprehensive rate testing (26–260 Mbps). The auto MCS selection ,due to this function is based on only signal strength, while the MCS should construct with modulation and coding scheme ,which the coding scheme is related with BER (bite error rate),previous vWiFI only implement random signal strength,meaning that bit error rate should also based on the channel state info.

jserv requested review from jychen0611 and rickywu0421 June 7, 2025 21:44

jserv requested changes Jun 7, 2025

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jychen0611 suggested changes Jun 8, 2025

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dingsen-Greenhorn force-pushed the dynamic_MCS branch from 79a5101 to 4e81b72 Compare June 8, 2025 15:33

jychen0611 suggested changes Jun 8, 2025

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jychen0611 reviewed Jun 10, 2025

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jychen0611 suggested changes Jun 12, 2025

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dingsen-Greenhorn force-pushed the dynamic_MCS branch 8 times, most recently from 48d23ac to 222746f Compare June 13, 2025 04:09

dingsen-Greenhorn force-pushed the dynamic_MCS branch from 222746f to 7033329 Compare June 13, 2025 17:50

dingsen-Greenhorn force-pushed the dynamic_MCS branch from 7033329 to a34bba7 Compare June 13, 2025 18:59

dingsen-Greenhorn force-pushed the dynamic_MCS branch 2 times, most recently from 5799346 to c556347 Compare June 13, 2025 19:12

dingsen-Greenhorn force-pushed the dynamic_MCS branch from c556347 to c96febb Compare June 13, 2025 19:24

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Implement dynamic MCS selection based on signal strength in vWIFI driver #80

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