[PATCH 21.02] ipq806x: backport cpufreq changes to 5.4

[Shane Synan]

[Since the mailing list didn't seem to pick up Ansuel's mobile reply,
I've reformatted here for easier future reference.]

On 7/20/21 8:36 PM EST, Ansuel Smith wrote:
> Sorry for the bad answer (I'm from phone)
> Anyway what you can try is tweak the OPP table and increase your CPU
> voltage...
> Don't know if it was related but some time ago I tested the router
> with overclock and strangely the router was more stable - 28 days of
> uptime while normally I have 3-4 days.
> My idea is some chip degradation or bad power supply and this cause
> crash.  Increasing the voltage seems to fix the problem.

Thank you!  In the upcoming weeks, I'll look into tinkering with the
voltage settings for the CPU in the OPP table.

With your advice in mind, I'm also suspecting it may be the combination
of CPU power draw and my USB 3.0 HDD's power drain (an old 1 TB Seagate
SRD00F1), as the latter is bus-powered rather than using an external
power supply.  I've acquired a digital USB test load and a USB 3.0
voltage/current meter is on the way (estimated arrival August 15th).
With any luck, I hope to reliably recreate the issue by running a
CPU-intensive task combined with a fixed current load to mimic the HDD
drawing from the NBG6817's USB 3.0 port, instead of the convoluted
Deja Dup SFTP backup test that I've been doing.

I'll also look at measuring the DC side of the power supply while it's
under load.  It might even be a combination of the bus-powered USB 3.0
HDD, chip degradation, and/or failing power supply.  I've had the
NBG6817 as my primary home router since November 2019 (bought new).

Again, thank you for your patience and time spent on this tricky issue!
I'll follow up if I uncover anything of interest, and I'll keep updating
folks in the OFTC/#openwrt-devel IRC channel too.

~ Shane