Hm, do we have a sane way to get an unmolested RX timestamp?
like, is there any twiddle we can do to use the second TSF on Kiwi and
later and leave it free-running, so in STA mode we can get the RX
timestamps from said free-running clock, rather than the hostap BSS
adjusted TSF?
-a
On 14 February 2014 02:50, Felix Fietkau <n...@openwrt.org> wrote:
> 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
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