Add more information about DVFS

[WizardTim]

Added some useful information about DVFS that was hidden away in some forum posts.

[lurch]

ping @pelwell and @popcornmix for confirmation.

[pelwell]

That works for me.

[JamesH65]

LGTM.

[lurch]

LGTM

[pelwell]

What is dtfs? I take it you mean dvfs? Where do you get the idea that our Device Tree is controlling DVFS in some way?

[JamesH65]

Those config.txt entries are unrelated to device tree.

[WizardTim]

You don't need to set dvfs=1 to change the CPU governor. The 'powersave' governor just limits the CPU frequency to the default idle 600 MHz (on RPi4). I have found a small difference in power consumption at idle between 'ondemand' and 'powersave' to be < 0.2 W.

Using dvfs=1 in addition I got a further reduction of 0.03 to 0.05 W at idle, but at 1.8 GHz OC I got a reduction of 0.36 W (+75 mV vs +15 mV core voltage offset).

• Tests done on a Raspberry Pi CM4 with headless Raspbian Lite install.

[aallan]

I know this was an existing reference to Wikipedia. But going forward post-#1911 I'm going to be stripping these out. Wikipedia is not a valid reference source.

[JamesH65]

It is on many subjects. Certainly that article is fine.

[aallan]

Right now, yes. But after someone vandalises the page or someone comes and makes major edits that are just wrong. Potentially not.

[JamesH65]

The schematics vs Wiki stripping is easy to explain - done by completely different people/teams. So its not a priority thing, its a who has time to do stuff thing. But not, we don't supply full schematics, and never have. and have no plans at present to do so.

[WizardTim]

@seamusdemora from what I've gathered:

• dvfs=0 - DFS only, core voltage is fixed for highest clock speed even at idle
• dvfs=1 - Allows under-voltage at low clocks and over-voltage at high clocks, default offset is -25 mV at idle (0.835 V)
• dvfs=2 - Disallows any under-voltage but over-voltage at higher clocks is still permitted

I believe the mapping between frequency and voltage is unrelated to which governor you select but it can be changed with over_voltage=0x#####.

[JamesH65]

Indeed, DVFS was implemented to keep the heat down on the Pi4, and it does a really good job, you can run fairly intensive tasks without too much throttling, as long as the duty cycle isn't too insane.

Note that we will always go for reliability/temperature control over the need to allow overclocking, so some changes we make might make overclocking less good, which I think has happened once in the past. But we do try to keep backwards functionality where possible.

[lurch]

See https://www.raspberrypi.org/blog/thermal-testing-raspberry-pi-4/ for more background info (but bear in mind it was written over 18 months ago, so things may have changed further since then).