Re: RFC: device thermal limits represented in device tree nodes

[Eduardo Valentin <eduardo.valentin@ti.com>]

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On 23-07-2013 21:44, Stephen Warren wrote:
> On 07/22/2013 07:25 AM, Eduardo Valentin wrote:
>> Hello Grant and Rob,
>>
>> (Resending, as I got a message saying: 
>> <devicetree-discuss@lists.ozlabs.org>: Recipient address rejected:
>> User has moved to devicetree at vger.kernel.org)
>>
>> I am writing this email to you specifically to ask your technical 
>> assessment with respect to representing device thermal limits as
>> device tree nodes. I am proposing to introduce device tree nodes to
>> describe these limits as thermal zones, their composition and their
>> relations with cooling devices and other thermal zones (thermal
>> data).
> 
> Given:
> https://lkml.org/lkml/2013/7/20/69
> [PATCH 3/3] MAINTAINERS: Refactor device tree maintainership
> 
> I'm explicitly CCing a few people besides Grant/Rob, and qouting the
> whole email.
> 

OK. Cool. In case thermal limits specification finds its way into DT, I
am willing to volunteer to be maintainer for the resulting bindings, in
case there is this need.

>>From my perspective, the concept of including thermal limits in DT
> seems reasonable, although I haven't looked at the proposed binding
> itself in detail yet.

Yeah, thanks Warren for your support.


> 
>> As you should know, device thermal limits are part of hardware 
>> specification. Considering your board layout, mechanics, power 
>> dissipation and composition of ICs, etc, that will impose thermal 
>> requirements on your system, and infringing these limits can lead
>> to device damage, device life time reduction or even end user harm.
>> Thus, the thermal data help to describe the hardware limits and
>> what needs to be done if those limits are crosses, as part of your
>> board design and non-functional requirements. Obviously that is
>> very dependent on your hardware, and not all of them will have
>> these non-functional requirements. Besides, describing these limits
>> has *nothing* to do with how you actually find these limits.
>>
>> In any case, there is a need to properly represent these
>> requirements and I am proposing to have this representation in
>> device tree. There were already couple of counter-arguments
>> claiming this is actually about configuration and performance
>> profile description. But I still stand against these two readings
>> of this proposal and again state that if one interprets it as
>> configuration or performance profile, that is a mis-understanding
>> [0]. Let me state it clear (again [1]), my proposal is to describe
>> hardware thermal limits, because these limits are part of a 
>> hardware specification; representing in device tree would not
>> infringe the original purpose of this data structure  ("The Device
>> Tree is a data structure for describing hardware."[2]).
>>
>> Before I explain my proposal, I want to highlight also that these
>> data is represented elsewhere already and it is reused across
>> different OS's. Thermal data is described using ACPI [3] and
>> operating systems ACPI-aware do support the interpretation of
>> thermal data. Linux is one example of such systems (I believe I do
>> not need to enlist here all systems supporting ACPI). On the other
>> hand, not all systems have ACPI or are specified to use ACPI.
>> Thus, here is another reason to represent properly thermal data, so
>> that we can scale across systems.
>>
>> In the specific case of Linux, the common thermal concepts between
>> ACPI systems and non-ACPI systems have been represented in the
>> thermal framework (CONFIG_THERMAL). Today, on ACPI systems, thermal
>> data is fetched from bootloader with help from the common ACPI
>> parser. For non-ACPI systems, the thermal data is actually coded as
>> part of device drivers.
>>
>> So, to the point, a brief explanation of my proposal goes as
>> follows: i   - trip points: a node to describe a point in the
>> temperature domain in which the system has to take an action. This
>> node describes just the point, not the action. Properties here are
>> temperature, hysteresis, and type (critical, hot, passive, active,
>> etc). ii  - binding parameters: the bind_param node is a node to
>> describe how actions (cooling devices) get assigned to trip points.
>> Cooling devices are expected to be loaded in the target system.
>> Properties here are: cooling device name, weight, trip_mask and
>> limits. iii - thermal zones: the thermal_zone node is the node
>> containing all the required info for describing a thermal zone with
>> hardware thermal limitation, including its bindings with cooling
>> devices. Properties here are:  type, passive_delay, polling_delay,
>> governor. The thermal_zone node must contain, apart from its own
>> properties, one node containing trip nodes and one node containing
>> all the zone bind parameters.
>>
>> Here is an example (on OMAP4430): thermal_zone { type = "CPU"; mask
>> = <0x03>; /* trips writability */ passive_delay = <250>; /*
>> milliseconds */ polling_delay = <1000>; /* milliseconds */ governor
>> = "step_wise"; trips { alert@100000{ temperature = <100000>; /*
>> milliCelsius hysteresis = <2000>; /* milliCelsius */ type =
>> <THERMAL_TRIP_PASSIVE>; }; crit@125000{ temperature = <125000>; /*
>> milliCelsius hysteresis = <2000>; /* milliCelsius */ type =
>> <THERMAL_TRIP_CRITICAL>; }; }; bind_params { action@0{ 
>> cooling_device = "thermal-cpufreq"; weight = <100>; /* percentage
>> */ mask = <0x01>; /* no limits, using defaults */ }; }; };
>>
>> In this current proposal, a 'thermal_zone' node would be embedded
>> inside a temperature sensor node, for simplicity. But other
>> possible builds could embedded them in the device with thermal
>> limits (CPU nodes, for instance) or they could be not embedded in
>> any specific node.
>>
>> A full documented description can be found here [4]. Also a branch 
>> containing: (a) needed changes in order to have this DT parser; (b)
>> the DT parser with documentation (c) examples on how drivers could
>> be changes to use the parser can be found in my branch here [5]. I
>> wrote the thermal DT parser to build thermal zones with the thermal
>> framework API. However, if one does not want to do that, it can
>> simple do not include a CONFIG_THERMAL_OF=y in her/his build, and
>> the calls will be translated to nops, and the device tree thermal
>> data can be parsed to somewhere else interested (other subsystem or
>> even user land). A TODO on this implementation is that it still
>> lacks the representation of thermal zones composed by several
>> sensors. However, I believe it is better to take an incremental 
>> approach here.  This series can already be used to improve most of
>> the existing platform thermal drivers (most are CPU thermal
>> drivers) and to reuse the existing code of some hwmon sensors to
>> build thermal zones for board thermal requirements.
>>
>> I have already posted a patch series with this proposal on [6],
>> that contains a reference for the original RFC. But looks like my
>> messages got moderated on device tree mailing list. Obviously,
>> within PM forum, feedback was quite positive. However, we cannot
>> proceed without proper assessment of other subsystems. lm-sensors
>> folks (Guenter) seam to be strongly against this series, as there
>> is a fear that this may introduce a mis-usage of DT. I still
>> believe this is needed for  hardware description, and thus not a
>> infringement on DT purposes.
>>
>> Please let me know your thoughts on this topic and apologize me if
>> my previous messages on this topic did not reach you (hope they
>> reach now).
>>
>> All best,
>>
>> Eduardo Valentin
>>
>> [0] - https://lkml.org/lkml/2013/7/17/621 [1] -
>> https://lkml.org/lkml/2013/7/18/279 [2] - www.devicetree.org [3] -
>> http://www.acpi.info/ [4] - 
>> https://git.kernel.org/cgit/linux/kernel/git/evalenti/linux.git/diff/Documentation/devicetree/bindings/thermal/thermal.txt?h=thermal_work/thermal_core/dt_parser&id=405bf0b51457ed055a082af2653d7ce757bc2e91
>>
>>
> [5] -
>> https://git.kernel.org/cgit/linux/kernel/git/evalenti/linux.git/log/?h=thermal_work/thermal_core/dt_parser
>>
>>
> [6] - https://lkml.org/lkml/2013/7/17/923
>>
>>
> 
> 
> 


-- 
You have got to be excited about what you are doing. (L. Lamport)

Eduardo Valentin


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