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From: Tyler Hall <tylerwhall@gmail.com>
Date: Wed, 25 Feb 2015 17:39:45 -0500
Message-ID: <CAOjnSCZJiREja3JWvhmTi+rqRCkm8+bhZ+YQifWXB8VSxQwFHQ@mail.gmail.com>
Subject: Re: [PATCH] thermal: armada: read stable temp on Armada XP
To: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>,
        linux-pm@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Zhang Rui <rui.zhang@intel.com>,
        Eduardo Valentin <edubezval@gmail.com>
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Gregory,

I instantiated both configurations and verified I get essentially the
same temperature from both, with the original register having a
slightly larger spread. I didn't reproduce any outliers in this run,
but I'll keep it running for a while to see if any occur.

I'm running this to gather samples.
# while true; do echo $(</sys/class/thermal/thermal_zone0/temp),
$(</sys/class/thermal/thermal_zone1/temp); done

https://i.imgur.com/c5wfPuy.png

I just got some information from our vendor indicating there's an
erratum for the register I'm proposing to use. The workaround is to
read repeatedly until the value is consistent. I still have open
questions:
1. Why is the current register exhibiting behavior that would be
described by the erratum while the proposed register seems more
reliable?
2. What is the underlying hardware difference between the two?

This is my hardware revision, though the registers in question appear
in the functional spec for Armada XP, so I would think they would
apply to all flavors.
mvebu-soc-id: MVEBU SoC ID=0x7846, Rev=0x2

Hopefully either of our queries will be answered and we can make a
more informed decision on this.

Thanks,
Tyler

On Wed, Feb 25, 2015 at 2:47 PM, Tyler Hall <tylerwhall@gmail.com> wrote:
> Hi Gregory, Eduardo,
>
> On Wed, Feb 25, 2015 at 05:10:14PM +0100, Gregory CLEMENT wrote:
>> By using it by default do you mean removing marvell,armadaxp-thermal
>> and adding armadaxp-filtered-thermal instead ?
>
> Yes, replacing it in device tree. For me,
> /sys/class/thermal/thermal_zone0/temp returns the same temperature but
> with less variability between samples. My intent was to switch the
> source of the data and not affect user space other than providing a
> more stable reading.
>
>> Tyler,
>> In the meantime could you double check your values? The temperature on my board
>> seemed broken on my board. If needed I can check on an other board. By the way
>> on which board/product did you try it?
>
> I'm on a custom board with the 4-core MV78460. In addition to my
> patch, this is new device tree entry.
>
>                         thermal@184c4 {
>                                compatible = "marvell,armadaxp-filtered-thermal";
>                                reg = <0x184c4 0x4
>                                         0x184d0 0x4>;
>                                 status = "okay";
>                         };
>
> I dumped some raw samples of the temperature fields in each of these
> registers. This CSV contains the raw values converted to decimal.
> http://pastebin.com/Umc3cy5L
> The first column is the current register at 0x182b0 27:19 and the
> second is the register at 0x184c4 9:1.
>
> Here's a quick plot of a larger sample size while the temperature was rising.
> https://imgur.com/E9HlSBx
> The blue value is the current 0x182b0 register which seems to bounce
> around the green value from 0x184c4.
>
> I'll try a test of instantiating both at the same time and comparing
> the final output of the driver.
>
> On Wed, Feb 25, 2015 at 1:39 PM, Eduardo Valentin <edubezval@gmail.com> wrote:
>>> Does that new thermal sensors only improve the stability or does it
>>> also modify the value?
>>>
>>> In the second case it will more or less break the user space expectation.
>>
>> Yeah, I agree here. We need to understand if this is:
>> (1) a fix of which register to use. In that case, the old dtbs are
>> essentially wrong, and the driver would need to adapt, I would say.
>> (2) a way to report better temperature extrapolations. then, this is
>> essentially a new temp sensor, and we should have two separated
>> compatible. in other words, just keep the patch the way it is.
>
> This *might* be a different physical sensor, or it could be from the
> same source with some averaging/filtering applied in hardware. The
> conversion formula is the same, however, and I get similar raw values
> from both.
>
>> Yes. Typically I ask to see the complete series, even if I am not taking
>> the DTS changes. That is, you send a complete series, with changes in
>> the kernel code and in the DTS, copying the required audience (from
>> kernel side and from DT side). Once the changes are accepted, the
>> patches will be picked by the correct tree maintainer. It is common that
>> DTS changes go via the platform tree, to avoid conflicts though.
>
> Thanks for the clarification. I'll send both in the next version as I
> suspect there will be a v2 of this change.
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