[ath9k-devel] Interrupt mitigation for USB Atheros dongles

[Felix Fietkau <nbd@openwrt.org>]

On 2014-02-14 02:45, Dimosthenis Pediaditakis wrote:
> Hi all,
> I am using a capacity and available bandwidth measurement software that
> compute estimates based on the gaps among packet trains/pairs.
> 
> For that reason I need to disable RX/TX interrupt mitigation for the
> devices that I take the measurement form, because it introduces skew,
> and renders unmeasurable the fast links (> 54Kbps).
> The less latencies the drivers+kernel introduces, the better.
> 
> I have so far managed to disable all together the RX mitigation for
> mini-PCIe devices by hard-coding  the following line in
>  "linux/drivers/net/wireless/ath/ath9k/ hw.c"
> ah->config.rx_intr_mitigation = false;
It seems to me that a better way to do this would be to drop your
fragile software based time measurement of rx interrupts, and instead
use the hardware timestamp (measured in microseconds).

> While the above trick improves my results, it doesn't change the
> behaviour of the Atheros-based USB WiFi dongles.
> AFAIK my TP-Link TL-WN722N USB dongle uses the ath9k_htc driver, but I
> haven't found anything relevant in the respective sources ( htc_*.{c, h} )
There is no interrupt mitigation here.

> Apart from that, I suspect that the USB-net driver itself might also
> introduce some fixed delays which affect the minimum time-spacing
> measurement that my software can perceive at the receiver side.
USB in general adds too much latency for what you want to do.
Use hardware timestamps here as well.

- Felix