After doing a bit more testing, doesn’t seem as simple as my last post, unfortunately. Found a third charger, from which the laptop currently (at 94% charge) draws ~2.2A at 19.2V (measured at laptop side); it does not trigger throttling. When connected to the PowerPort Atom, which triggers throttling, at 94% charge, the laptop draws ~1.8-1.9A at 19.4V, yet still throttles (to around 3.2GHz v.s. ~4.5GHz on cpu0 when running
taskset 1 stress -c 1 (i.e. single-threaded load pinned to CPU0)).
I can just watch the laptop’s CPU frequency (
watch -n 0.1 cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq) and swap between the two chargers: plug in the PinePower or the third (unbranded) charger, see the CPU frequency quickly stabilize at 4.5-4.6GHz. Unplug, CPU frequency drops (due to TLP changing the governor to powersave, this shouldn’t matter); plug the Atom PD4 in, CPU frequency also quickly stabilizes at ~4.5GHz for just a second and then drops to something closer to 3.5GHz.
Swapping cables between the two chargers had no effect.
Next theory: EMI. Unfortunately, my good RF equipment isn’t accessible right now, but I have a 200MHz scope, so I put that on a test point on my USB-C tester (which is connected directly to Vbus; the tester was on the laptop side for all of these) to take a look at noise.
Here’s the Atom PD4 (triggers throttling) v.s. the PinePower (doesn’t)-
Nothing obviously sticks out, as far as I can tell (but I’m no engineer); though the Atom does seem to have more RMS noise power…
Attaching a ferrite bead to the USB-C cable did not affect the throttling behavior.
If there’s EMI going on, it seems as though it’d have to be internal- not as simple as just a noisy adapter. But then why can one adapter trigger it and another not? Very strange.