Re: [PATCH net-next v2] usbnet: Add support for Byte Queue Limits (BQL)

[Eric Dumazet]

On Thu, Nov 6, 2025 at 9:56 AM Simon Schippers
<simon.schippers@xxxxxxxxxxxxxx> wrote:
>
> In the current implementation, usbnet uses a fixed tx_qlen of:
>
> USB2: 60 * 1518 bytes = 91.08 KB
> USB3: 60 * 5 * 1518 bytes = 454.80 KB
>
> Such large transmit queues can be problematic, especially for cellular
> modems. For example, with a typical celluar link speed of 10 Mbit/s, a
> fully occupied USB3 transmit queue results in:
>
> 454.80 KB / (10 Mbit/s / 8 bit/byte) = 363.84 ms
>
> of additional latency.
>
> This patch adds support for Byte Queue Limits (BQL) [1] to dynamically
> manage the transmit queue size and reduce latency without sacrificing
> throughput.
>
> Testing was performed on various devices using the usbnet driver for
> packet transmission:
>
> - DELOCK 66045: USB3 to 2.5 GbE adapter (ax88179_178a)
> - DELOCK 61969: USB2 to 1 GbE adapter (asix)
> - Quectel RM520: 5G modem (qmi_wwan)
> - USB2 Android tethering (cdc_ncm)
>
> No performance degradation was observed for iperf3 TCP or UDP traffic,
> while latency for a prioritized ping application was significantly
> reduced. For example, using the USB3 to 2.5 GbE adapter, which was fully
> utilized by iperf3 UDP traffic, the prioritized ping was improved from
> 1.6 ms to 0.6 ms. With the same setup but with a 100 Mbit/s Ethernet
> connection, the prioritized ping was improved from 35 ms to 5 ms.
>
> [1] https://lwn.net/Articles/469652/
>
> Signed-off-by: Simon Schippers <simon.schippers@xxxxxxxxxxxxxx>
> ---

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>