Re: mac80211 HT/VHT MCS check change causing Wi-Fi speed drop (~20 Mbps cap) on iwlwifi

[Johannes Berg <johannes@sipsolutions.net>]

On Fri, 2026-04-24 at 04:28 -0400, Jayden Fowler wrote:
> Hello,
> 
> I think I’ve run into a regression in mac80211 that affects Wi-Fi speeds 
> on Intel iwlwifi (and possibly some Realtek cards too).
> 
> After testing a bunch of kernels, I noticed something pretty consistent:
> 
> Kernels with this commit Wi-Fi gets stuck around ~20 Mbps max
> Kernels without it (or after removing a small part of it) speeds go back 
> to normal (hundreds of Mbps as expected)
> 
> The commit I bisected it to is:
> 
> 574faa0e936d12718e2cadad11ce1e184d9e5a32
> "wifi: mac80211: add HT and VHT basic set verification"
> 
> The issue seems to come from the new HT/VHT MCS verification logic. When 
> it runs, it sometimes decides the AP’s advertised rates are 
> “unsupported” and then forces a downgrade in connection mode/bandwidth.
> 
> That downgrade looks like it’s what causes the big speed drop.

Yeah, there are too many AP bugs ...

https://patchwork.kernel.org/project/linux-wireless/patch/99Mv9QEceyPrQhSP52MtAVmz0_kWJmzqotJjD9YW6LGLqk-AZloAueUyHCURilFkuqOh6Ecv8i2KKdSE1ujP3AnbU5QEouVisT1w_V3xdfc=@r26.me/

johannes