5GHz inbound connection latency spikes on Raspberry Pi

[Nicolas Saenz Julienne]

Hi all,
I'm seeing latency spikes on inbound 5GHz connections with brcmfmac on RPi4.
For example, this is me pinging an idle RPi4 running today's linux-next build
from my work computer (although I can reproduce this with older kernels and/or
RPi3):

	nico at linux-9qgx:~> ping 192.168.1.18
	PING 192.168.1.18 (192.168.1.18) 56(84) bytes of data.
	64 bytes from 192.168.1.18: icmp_seq=1 ttl=64 time=306 ms
	64 bytes from 192.168.1.18: icmp_seq=2 ttl=64 time=1351 ms
	64 bytes from 192.168.1.18: icmp_seq=3 ttl=64 time=326 ms
	64 bytes from 192.168.1.18: icmp_seq=4 ttl=64 time=175 ms
	64 bytes from 192.168.1.18: icmp_seq=5 ttl=64 time=1431 ms
	64 bytes from 192.168.1.18: icmp_seq=6 ttl=64 time=425 ms
	64 bytes from 192.168.1.18: icmp_seq=7 ttl=64 time=1457 ms
	64 bytes from 192.168.1.18: icmp_seq=8 ttl=64 time=441 ms
	64 bytes from 192.168.1.18: icmp_seq=9 ttl=64 time=360 ms
	64 bytes from 192.168.1.18: icmp_seq=10 ttl=64 time=1395 ms
	64 bytes from 192.168.1.18: icmp_seq=11 ttl=64 time=388 ms
	64 bytes from 192.168.1.18: icmp_seq=12 ttl=64 time=1431 ms
	64 bytes from 192.168.1.18: icmp_seq=13 ttl=64 time=416 ms
	64 bytes from 192.168.1.18: icmp_seq=14 ttl=64 time=1272 ms
	64 bytes from 192.168.1.18: icmp_seq=15 ttl=64 time=257 ms
	64 bytes from 192.168.1.18: icmp_seq=16 ttl=64 time=1291 ms
	64 bytes from 192.168.1.18: icmp_seq=17 ttl=64 time=277 ms

It turns out that if I disable the low power mode on the wireless network
interface: 'iwconfig wlan0 power off', everything behaves as expected, with
latencies between averaging 10ms. I've also found this patch[1] from RPi's
downstream kernel tree which tweaks the LPM entry delay and makes things a
little better. With it, SSHing into the machine is bearable.

I can understand that tweak not being OK for all brcmfmac users so I figured I
could apply it conditionally on RPi's compatible strings. That said, before
doing anything I wonder if you have a better solution in mind.

Also note that latencies on 2GHz connections are also visible, but in the order
of 150-200ms, which is more acceptable.

Regards,
Nicolas

[1] 
https://github.com/raspberrypi/linux/commit/3e0d7cb39d3229a00f06de5b3e60bd7acf0ece3f#diff-1b8fac2d29272755e8f53049866fc1e5

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