Call __WFI() when going idle

[betzw]

Description

• Type: Enhancement
• Priority: Minor

---

Enhancement

As long as there is no tickless mode, function default_idle_hook() should (at least) call __WFI() to reduce power consumption.

Target
all

Toolchain:
all

mbed-cli version:
all

Expected behavior
Reduced power consumption when idle.

Actual behavior
No reduction in power consumption when idle.

---

Enhancement

Suggested enhancement
Call __WFI() inside function default_idle_hook() in non-tickless mode.

[sg-]

@betzw can you point at the particular change? bdab10d is quite large

[betzw]

It is not related to any particular change since it seems as if this behavior exists since a long time.
Basically it is about commenting the call to sleep()out in function default_idle_hook() (see here).
Not sure how to interprete the comments above the commented sleep(), but I think that in an officially released OS a non-debug build should put the system in at least __WFI() when its going idle.

[0xc0170]

Tickless mode shall get into the code base this year, there were some problems as you might have noticed with tickers that needs fixing, it's parked on the feature branch at the moment.

Even if you add __WFI() how much would we save as systick is running?

[betzw]

No, I didn't notice the issues with tickers. But did I get it right that when calling __WFI() in some situations tickers might not behave correctly anymore? If yes, obviously you shouldn't call __WFI() for the moment, otherwise even if it will save less than in tickless mode, it will anyway improve power consumption.

[betzw]

Any news on this @sg-, @0xc0170?

[betzw]

Ping!

[0xc0170]

There's feature branch, that needs more work: https://github.com/ARMmbed/mbed-os/tree/feature_rtos_tickless. It should resolve this.

cc @bulislaw

[betzw]

Seems like an abandoned branch, I mean last commit about 4 months ago. Have you any timeline for when this branch would be merged?

[betzw]

@screamerbg

[bulislaw]

There were difficulties with the tickless mode due to differences in platforms and how they implement low power timer. I have an idea how to address it, I'll write it down over Christmas and then we can discuss it and find some time to implement it.

[0xc0170]

Seems like an abandoned branch, I mean last commit about 4 months ago. Have you any timeline for when this branch would be merged?

Addition to the comment above about issues with the tickers, there's a plan for RTX update, therefore this idle loop work should be based on that update (version >5.4)

[bulislaw]

That shouldn't be a problem, as the RTX API changes are cosmetic.