[PATCH v2] imx: thermal: use CPU temperature grade info for thresholds

[Jon Nettleton]

On Sun, May 24, 2015 at 4:48 AM, Shawn Guo <shawn.guo at linaro.org> wrote:
> On Thu, May 21, 2015 at 04:45:47PM -0700, Tim Harvey wrote:
>> The IMX6Q/IMX6DL SoC's have a 2-bit temperature grade stored in OTP which
>> is valid for all IMX6 SoC's (despite the fact that the IMXSDLRM and
>> IMXSXRM do not document this - this has been proven via tests as well as
>> verified by Freescale FAE).
>>
>> Instead of assuming a fixed 85C for passive cooling threshold and 105C for
>> critical use the thermal grade for these configurations.
>>
>> We will set the critical to maxT - 5C and passive to maxT - 10C.

I would like to chime in here if you don't mind.  I have been carrying
a patch similar to this in the SolidRun repo to fix cooling issues
that we have had.  I would recommend keeping the passive temp at maxT
- 20C due to the thermal properties of the chip.  I have found that
around 85-90C we can maintain a relatively steady thermal state with
only passive cooling.  Generally with a hard non-NEON based cpu
workload the iMX6 will level off at about 87C with all the cores
clocked to 1Ghz, and sometimes dipping down to 800Mhz periodically.
With a NEON based workload on all the cores it will push beyond this
and generally end up finding steady state at about 800Mhz right around
90C.

If you raise the initial passive threshold by 10C it will allow enough
heat to build up in the chip that the only way to avoid reaching
critical temps is by dropping the CPU down to its lowest frequency.
This is not the best experience as then you have a much warmer chip
and if the workload doesn't change you will just be switching between
running at the highest cpu frequency or lowest which makes for a
choppy experience.  A longer passive cooling zone allows the
temperature of the chip to be regulated using only passive methods but
without drastic performance drops.

I am doing things a bit differently in my implementation as I setup a
passive cooling zone for each cpu frequency, but that is just so you
can have more control from userspace by changing the different passive
trip points.

-Jon