The Linux kernel does all the PM stuff nowadays automatically afik. At least I didn't touch any of this settings for a long time and it just works. When not compiling anything or something that like, but surfing, editing text (in something besides IDEA), watching videos, or such "light tasks" the computer has only a few percent CPU load at max, the fan is off, it has room temperature, and runs for hours on battery.
I would also support what one of the siblings said: Explicitly pining the CPU to lower frequencies doesn't help much. For example web pages load much longer. I think a few hundred milliseconds at higher speed and then falling back into the idle mode doesn't cost much extra battery compared to up a few seconds of full CPU load at lower frequencies for a page load.
Yes, it safes battery to make the CPU slow. But than doing anything with the computer is also much slower and everything takes "forever" therefore. I'm not sure this is a net win when the battery lasts longer but you waste much more time waiting. A super slow computer feels worse in the end, imho.
It's just an interesting observation, I'm not really preaching any huge benefits here. When you tell the systemd power manager to enter max-efficiency mode it runs an interactive desktop in suspend-level frequencies. That's all I find interesting here.
Running the CPU constantly at 400MHz is likely to consume more power than bursty workloads at full speed. There are various linked clock domains, and if the CPU cores can't get into low power states then neither can other bits of hardware on the SoC.
Who says the CPU is running constantly at 400MHz? I assume for video to work at all like that it must by bypassing the CPU, and text editing would look to the machine like "all CPU cores idle... waiting... waiting... oh, the user pressed a button, let's handle that for 50ms, okay now we can idle while we wait for the slow human to press another key..."
I'm familiar with the race-to-idle theory, but even on the lowest power mode it will scale between 400-900mhz depending on the workload. If there was anything blocking the CPU or hogging cycles, it would be clocking a bit faster.
That being said, it's still not a huge power gain. I mostly do it to keep the system cool and the fan off.
