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

[Jon Nettleton]

Tim,

Okay that all sounds fine.  I came into this half way through so I
missed the core of what you were trying to accomplish, sorry to
de-rail the conversation a bit.

I will gladly ACK the patch and will follow up with patches we can
discuss for additional changes.

-Jon

On Tue, Jul 28, 2015 at 4:50 PM, Tim Harvey <tharvey at gateworks.com> wrote:
> On Tue, May 26, 2015 at 11:08 PM, Jon Nettleton <jon.nettleton at gmail.com> wrote:
>> On Tue, May 26, 2015 at 11:24 PM, Tim Harvey <tharvey at gateworks.com> wrote:
>>> On Sat, May 23, 2015 at 10:19 PM, Jon Nettleton <jon.nettleton at gmail.com> wrote:
>>>> 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
>>>
>>> Jon,
>>>
>>> I can agree with leaving a Max-20C passive delta. What do you think
>>> about the critical threshold of Max-5C and rule of not allowing it to
>>> be changed?
>>>
>>
>> Tim,
>>
>> I definitely agree that the Critical temp should be a fixed point.  Is
>> the purpose of lowering the critical threshold from the hardware
>> default, to allow Linux to shutdown more cleanly rather than just have
>> the hardware shutting down?  If that is the case then I think that is
>> fine.  If it is to protect the SOC then that is unnecessary.  We have
>> heated the SOCs to well beyond the critical threshold and they have
>> survived just fine.
>>
>> This is a bit out of context but here is the formula I am using to
>> figure out my trip points.  By default I use a linear set of trip
>> points for passive cooling.
>> https://github.com/linux4kix/linux-linaro-stable-mx6/commit/212c17d543739a5fe0bd75b66c10f05177e8bcb0
>>
>> The short of it is I set a trip delta of 6C and then figure out the
>> lowest passive trip point as Critical - (#passive trip points * trip
>> delta), where each cpu frequency stage is a passive trip point.  This
>> will allow an 800Mhz SOC with 2 trip points to run at full speed
>> longer than a 1.2Ghz with 4 trip points.  The idea being that the
>> higher the clock rate means we will generate more heat and have more
>> passive cooling levels so it is better to drop the top speed of the
>> CPU earlier in order to let the passive cooling be effective and find
>> a steady state.
>>
>> This may be a bit over the top but has fixed problems where long
>> running processes would build up heat and eventually cause a thermal
>> shutdown, but doesn't completely cripple the faster SOCs.
>
> Jon,
>
> Yes - the purpose of lowering the critical threshold from the hardware
> default is to allow Linux to shutdown more cleanly.
>
> If you agree with the fact that the patch here offers the improvement
> of using OTG temperature grade as a basis can you ack it and if you
> feel that the thresholds need to be adjusted perhaps propose a
> follow-on patch? I feel people can debate the temperature delta's
> endlessly but what I was really after here was to fix the fact that
> all the processors are not temperature graded equally because they are
> packaged differently (metal case on automotive offering better thermal
> conductivity vs plastic case on consumer)
>
> Regards,
>
> Tim