Re: [linux-pm] [RFC] the generic thermal layer enhancement

Eduardo Valentin
2012-05-30T10:30:43Z

Hello Rui,

Now I copied Amit, for real :-)

I like your proposal, some comments as follow.

On Wed, May 30, 2012 at 04:51:00PM +0800, Zhang Rui wrote:
> On 三, 2012-05-30 at 16:49 +0800, Zhang Rui wrote:
> > Hi, all,
> >
> > It is great to see more and more users of the generic thermal layer.
> > But as we know, the original design of the generic thermal layer comes
> > from ACPI thermal management, and some of its implementation seems to be
> > too ACPI specific nowadays.

Good. We have also basic OMAP support, on top of Amit's work. I sent
recently a very basic support. I will be pushing them, while they evolve.

> >
> > Recently I'm thinking of enhance the generic thermal layer so that it
> > works well for more platforms.

As you said, for non-ACPI support, the "generic" layer needs some
extension and refactoring to be more generic :-).

> >
> > Below are some thoughts of mine, after reading the patches from Amit
> > Daniel Kachhap, and ACPI 3.0 thermal model. Actually, I have started
> > coding some RFC patches. But I do really want to get feedback from you
> > before going on.

OK.

> >
> > G1. supporting multiple cooling states for active cooling devices.
> >
> > The current active cooling device supports two cooling states only,
> > please refer to the code below, in driver/thermal/thermal_sys.c
> > case THERMAL_TRIP_ACTIVE:
> > ...
> > if (temp >= trip_temp)
> > cdev->ops->set_cur_state(cdev, 1);
> > else
> > cdev->ops->set_cur_state(cdev, 0);
> > break;
> >
> > This is an ACPI specific thing, as our ACPI FAN used to support
> > ON/OFF only.
> > I think it is reasonable to support multiple active cooling states
> > as they are common on many platforms, and note that this is also
> > true for ACPI 3.0 FAN device (_FPS).
> >
> > G2. introduce cooling states range for a certain trip point
> >
> > This problem comes with the first one.
> > If the cooling devices support multiple cooling states, and surely
> > we may want only several cooling states for a certain trip point,
> > and other cooling states for other active trip points.
> > To do this, we should be able to describe the cooling device
> > behavior for a certain trip point, rather than for the entire
> > thermal zone.

For G1+G2, I agree with your proposal. I had some discussion with Amit
regarding this. In his series of patches we increase / decrease the cooling
device state linearly and steadily.

But if we would have what you are saying, we could bind cooling device
set of states with trip points.

> >
> > G3. kernel thermal passive cooling algorithm
> >
> > Currently, tc1 and tc2 are hard requirements for kernel passive
> > cooling. But non-ACPI platforms do not have this information
> > (please correct me if I'm wrong).
> > Say, for the patches here
> > http://marc.info/?l=linux-acpi&m=133681581305341&w=2
>
> Sorry, forgot to cc Amit, the author of this patch set.
>
> thanks,
> rui
> > They just want to slow down the processor when current temperature
> > is higher than the trip point and speed up the processor when the
> > temperature is lower than the trip point.
> >
> > According to Matthew, the platform drivers are responsible to
> > provide proper tc1 and tc2 values to use kernel passive cooling.
> > But I'm just wondering if we can use something instead.
> > Say, introduce .get_trend() in thermal_zone_device_ops.
> > And we set cur_state++ or cur_state-- based on the value returned
> > by .get_trend(), instead of using tc1 and tc2.

I fully support this option and could cook up something on this.
The TC1 and TC2 should go inside the .get_trend() callbacks for ACPI.
Should probably go away from the registration function that we have
currently.

We could have generic trending computation though. Based on timestamping
and temperature reads, and make it available for zones that want to used it.

> >
> > G4. Multiple passive trip points
> >
> > I get this idea also from the patches at
> > http://marc.info/?l=linux-acpi&m=133681581305341&w=2
> >
> > IMO, they want to get an acceptable performance at a tolerable
> > temperature.
> > Say, a platform with four P-states. P3 is really low.
> > And I'm okay with the temperature at 60C, but 80C? No.
> > With G2 resolved, we can use processor P0~P2 for Passive trip point
> > 0 (50C), and P3 for Passive trip point 1 (70C). And then the
> > temperature may be jumping at around 60C or even 65C, without
> > entering P3.

Yeah, I guess we need to solve G1+G2 first to allow this. But I also agree
that ideally, there should be possibility to have multiple passive trip points.

> >
> > Further more, IMO, this also works for ACPI platforms.
> > Say, we can easily change p-state to cool the system, but using
> > t-state is definitely what we do not want to see. The current
> > implementation does not expose this difference to the generic
> > thermal layer, but if we can have two passive trip points, and use
> > p-state for the first one only... (this works if we start polling
> > after entering passive cooling mode, without hardware notification)
> >
> > G5. unify active cooling and passive cooling code
> >
> > If G4 and G5 are resolved, a new problem to me is that there is no
> > difference between passive cooling and active cooling except the
> > cooling policy.

OK...

> > Then we can share the same code for both active and passive cooling.
> > maybe something like:
> >
> > case THERMAL_TRIP_ACTIVE:
> > case THERMAL_TRIP_PASSIVE:
> > ...
> > tz->ops->get_trend();

Would the get_trend take into account if we are cooling with active or passive
cooling device?

> > if (trend == HEATING)
> > cdev->ops->set_cur_state(cdev, cur_state++);
> > else if (trend == COOLING)
> > cdev->ops->set_cur_state(cdev, cur_state--);
> > break;

I believe we should have something for temperature stabilization there as well.

Besides, if we go with this generic policy, then the zone update would be much
simpler no?

> >
> > Here are the gaps in my point of view, I'd like to get your ideas about
> > which are reasonable and which are not.

Here are some other thoughts:
G6. Another point is, would it make sense to allow for policy extension? Meaning,
the zone update would call a callback to request for update from the zone
device driver?

G7. How do we solve cooling devices being shared between different thermal zones?
Should we have a better cooling device constraint management?

G8. On same topic as G7, how are we currently making sure that thermal constraints
don't get overwritten by, let's say, userspace APIs? I guess the generic CPU cooling
propose by Amit suffers of an issue. If user sets cpufreq governor to userspace
and sets the frequency to its maximum, say in a busy loop, the thermal cooling
could potentially be ruined.

G9. Is there any possibility to have multiple sensors per thermal zone?

G10. Do we want to represent other sensing stimuli other that temperature? Say,
current sensing?

G11. Do we want to allow for cross zoning policies? Sometimes a policy may use
temperature from different thermal zone in order to better represent what
is going on in its thermal zone.

> >
> > Any comments are appreciated! Thanks!

Thanks to you for starting this up! The above are points that come to my mind now.
I will keep updating the list if something else come to my mind.

> >
> > -rui
> >
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> > https://lists.linuxfoundation.org/mailman/listinfo/linux-pm
>
>
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All best,

---
Eduardo Valentin
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