Hi,

On Tue, Oct 6, 2020 at 3:24 PM Rob Herring [off-list ref] wrote:

On Fri, Oct 2, 2020 at 12:39 PM Doug Anderson [off-list ref] wrote:

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Hi,

On Fri, Oct 2, 2020 at 9:40 AM Lukasz Luba [off-list ref] wrote:

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On 10/2/20 4:47 PM, Doug Anderson wrote:

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Hi,

On Fri, Oct 2, 2020 at 8:13 AM Lukasz Luba [off-list ref] wrote:

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Hi Doug,

On 10/2/20 3:31 PM, Doug Anderson wrote:

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Hi,

On Fri, Oct 2, 2020 at 4:45 AM Lukasz Luba [off-list ref] wrote:

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Update the documentation for the binding 'sustainable-power' and allow
to provide values in an abstract scale. It is required when the cooling
devices use an abstract scale for their power values.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
---
   .../devicetree/bindings/thermal/thermal-zones.yaml  | 13 +++++++++----
   1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/Documentation/devicetree/bindings/thermal/thermal-zones.yaml b/Documentation/devicetree/bindings/thermal/thermal-zones.yaml
index 3ec9cc87ec50..4d8f2e37d1e6 100644
--- a/Documentation/devicetree/bindings/thermal/thermal-zones.yaml
+++ b/Documentation/devicetree/bindings/thermal/thermal-zones.yaml

@@ -99,10 +99,15 @@ patternProperties:
         sustainable-power:
           $ref: /schemas/types.yaml#/definitions/uint32
           description:
-          An estimate of the sustainable power (in mW) that this thermal zone
-          can dissipate at the desired control temperature. For reference, the
-          sustainable power of a 4-inch phone is typically 2000mW, while on a
-          10-inch tablet is around 4500mW.
+          An estimate of the sustainable power (in mW or in an abstract scale)
+         that this thermal zone can dissipate at the desired control
+         temperature. For reference, the sustainable power of a 4-inch phone
+         is typically 2000mW, while on a 10-inch tablet is around 4500mW.
+
+         It is possible to express the sustainable power in an abstract
+         scale. This is the case when the related cooling devices use also
+         abstract scale to express their power usage. The scale must be
+         consistent.

Two thoughts:

1. If we're going to allow "sustainable-power" to be in abstract
scale, why not allow "dynamic-power-coefficient" to be in abstract
scale too?  I assume that the whole reason against that originally was
the idea of device tree purity, but if we're allowing the abstract
scale here then there seems no reason not to allow it for
"dynamic-power-coefficient".

With this binding it's a bit more tricky.
I also have to discuss a few things internally. This requirement of
uW/MHz/V^2 makes the code easier also for potential drivers
like GPU (which are going to register the devfreq cooling with EM).

Let me think about it, but for now I would just update these bits.
These are required to proper IPA operation, the dyn.-pow.-coef. is a
nice to have and possible next step.

I guess the problem is that Rajendra is currently planning to remove
all the "dynamic-power-coefficient" values from device tree right now
and move them to the source code because the numbers we currently have
in the device tree _are_ in abstract scale and thus violate the
bindings.  Moving this to source code won't help us get to more real
power numbers (since it'll still be abstract scale), it'll just be
pure churn.  If we're OK with the abstract scale in general then we
should allow it everywhere and not add churn for no reason.

IIUC he is still going to use the Energy Model, but with different
registration function. We have such a driver: scmi-cpufreq.c, which
uses em_dev_register_perf_domain(). He can still use EM, EAS, IPA
not violating anything.

Right.  He's going to take the exact same "abstract scale" numbers
that he has today and take them out of device tree and put them in the
cpufreq driver.  Doing so magically makes it so that he's not
violating anything since "abstract scale" is not currently allowed in
device tree but is allowed in the cpufreq driver.  I'm not saying that
he's doing anything wrong, I'm just saying that it's pointless churn.
If we're OK with "abstract scale" in one place in the device tree we
should be OK with it everywhere in the device tree.  Then Rajendra
wouldn't need his patch at all and he could leave his numbers in the
device tree.

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The real problem that we want to address is with sustainable-power in
IPA. It is used in power budget calculation and if the devices operate
in abstract scale, then there is an issue.
There are two options to get that value:
1. from DT, which can have optimized value, stored by OEM engineer
2. from IPA estimation code, which just calculates it as a sum of
minimum OPP power for each cooling device.

The 2nd option might not be the best for a platform, so vendor/OEM
engineer might want to provide a better value in DT -> 1st option.
This is currently against the binding description and I have to fix it.

Right, things are already broken today because a SoC vendor could
(without violating any rules) provide their SoC core
"dynamic-power-coefficient" in "abstract scale" in code and there
would be no way to for a board to (without violating DT bindings)
specify a "sustainable-power".  ...so, in that sense, your patch does
provide a benefit even if we don't make any changes to the rules for
"sustainable-power".  All I'm saying is that if these new rules for
allowing an abstract scale for "sustainable-power" in the device tree
are OK that it should _also_ be OK to add new rules to allow an
abstract scale for "dynamic-power-coefficient".

Didn't we beat this one to death with "dynamic-power-coefficient"?

We did?  Where / when?  I'm not sure I was involved, but right now
both "sustainable-power" and "dynamic-power-coefficient" are still
defined in the device tree to be in real units, not abstract scale.
Are you saying that we beat it to death and decided that it needed to
be in real units, or we beat it to death and decided that abstract
scale was OK and we just didn't put it in the bindings?

That is the abstract scale because I don't think you can really ever
measure it

That's debatable.  it's not very hard to get reasonable measurements.
Matthias provided a recipe earlier in the thread.  See commit
ac60c5e33df4 ("ARM: dts: rockchip: Add dynamic-power-coefficient for
rk3288").  In that case he used a machine that could easily measure
power on the CPU rail, but if you simply keep all other rails in the
system constant (and/or run a long enough test), you can easily
accomplish this by just querying the smart battery in systems.

and because vendors don't want to advertise their absolute
power.

That is certainly true, though after a device has shipped it's not
that hard to measure.

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2. Is it worth adding some type of indication of what type of units
"sustainable-power" is represented in?  Maybe even a made up unit so
that you could tell the difference between made up units in the same
system?  I'd envision something like:

sustainable-power-units = "qualcomm,sc7180-bogoWatts"

...and on the dynamic-power-coefficient side, the same:

dynamic-power-coefficient-units = "qualcomm,sc7180-bogoWatts"

One could imagine someone even later (after devices are widely
distributed) figuring out translations between these bogoWatts numbers
and real Watts if someone could come up with a case where it matters.

To figure this out we don't need a new binding.
I think a simple comment in the DT would be enough for this, even e.g.:

sustainable-power = <100> /* bogoWatts */

There are some important differences:

a) Your comment is gone when the device tree is compiled.  If we
actually add a string to the device tree then, in theory, we can add
conversions in code (without touching the device tree) down the road.

We don't need code and binding with a bogoscale. It is up to the
platform integrator to make sure the scale in consistent in all devices.
Comment in DT is good enough.

One other nice thing about having the units is that the device tree is
supposed to be more of a "pure" thing, less sullied about what's
convenient and more about a real description of a thing.  Presumably
that's why "abstract scale" wasn't allowed originally?  In any case,
giving quantifiable units to the number somehow makes it feel less
made up because it's possible to come up with a way to convert it back
to real units.

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b) I believe there can be more than one abstract scale present in a
single device tree, at least in theory.  Adding a string allows you to
know if you're comparing apples to apples or apples to organges.

IMHO DT is not the place for such abstractions, but Rob might correct me
here.

Yup, seems like we're blocked waiting for Rob to chime in unless
someone else has the authority to make the call about how to deal with
"abstract scale" numbers in the device tree.

I don't really know nor completely follow the issues. I just get all
these PM related bindings piece by piece with everyone solving their
own single issue. It's death by 1000 cuts. So my default position is
NAK. All the missing pieces and deficiencies can build up until
there's a coherent picture (maybe?).

I'm totally confused.  NAK on what?  NAK on Lukasz's patch?  ...or
Lukasz's patch is totally fine but NAK on also allowing abstract scale
for 'dynamic-power-coefficient".  Or NAK on adding units?  NAK on
something else?

-Doug