RE: [PATCH v5] wifi: rtw89: usb: fix TX flow control by tracking in-flight URBs

[Ping-Ke Shih <pkshih@realtek.com>]

Lucid Duck <lucid_duck@justthetip.ca> wrote:
> rtw89_usb_ops_check_and_reclaim_tx_resource() returns a hardcoded
> placeholder value (42) instead of actual TX resource availability.
> This violates mac80211's flow control contract, preventing backpressure
> and causing uncontrolled URB accumulation under sustained TX load.
> 
> Fix by adding per-channel atomic counters (tx_inflight[]) that track
> in-flight URBs. Increment before usb_submit_urb() with rollback on
> failure, decrement in the completion callback, and return the
> remaining capacity to mac80211. The firmware command channel (CH12)
> always returns 1 since it has its own flow control.
> 
> The pre-increment pattern prevents a race where USB core completes the
> URB on another CPU before the submitting code increments the counter.
> 
> 128 URBs per channel provides headroom for RTL8832CU at 160 MHz
> bandwidth. Tested on RTL8852AU (USB3 80 MHz) where 64 and 128 showed
> equivalent throughput, and on RTL8832AU where 128 sustained full
> throughput under 8-stream parallel load.
> 
> Tested on D-Link DWA-X1850 (RTL8832AU), kernel 6.19.8, Fedora 43:
> 
>                      Unpatched -> Patched (128 URBs)
>   USB3 5GHz UL:      844 -> 837 Mbps (no regression)
>   USB3 5GHz retx:    3 -> 0
>   USB3 2.4GHz UL:    162 -> 164 Mbps (no regression)
>   4-stream UL:       858 -> 826 Mbps (within variance)
>   8-stream UL:       872 -> 826 Mbps (within variance)
>   UDP flood:         0% loss (690K datagrams)
>   60-second soak:    855 Mbps, 0 retransmits
> 
> Reported-by: morrownr <morrownr@gmail.com>
> Signed-off-by: Lucid Duck <lucid_duck@justthetip.ca>

Acked-by: Ping-Ke Shih <pkshih@realtek.com>