Date:	Wed, 24 Jul 2013 09:29:09 -0400
From:	Eduardo Valentin <eduardo.valentin@...com>
To:	Mark Rutland <mark.rutland@....com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Eduardo Valentin <eduardo.valentin@...com>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ian.campbell@...rix.com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Guenter Roeck <linux@...ck-us.net>,
	Durgadoss R <durgadoss.r@...el.com>,
	"Zhang, Rui" <rui.zhang@...el.com>, Wei Ni <wni@...dia.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RFC: device thermal limits represented in device tree nodes

On 24-07-2013 06:45, Mark Rutland wrote:
> On Wed, Jul 24, 2013 at 02:44:38AM +0100, 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@...ts.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.
>>
>> 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.
> 
> The concept of defining hard thermal limits in DT certianly seems
> reasonable.

Good.

> 
>>>From a quick look at the version on lkml [1], it seems like this leaks a
> Linux implementation details (e.g. governer names) into the binding, and
> I think that the linkage of devices to thermal zones should be definedd
> more explicitly. A reposting of the series to devicetree (and lakml?)
> would be helpful for review.
> 

On governor names, here are different approaches:
(a) - name the property 'policy' and let OS decide to interpret it.
(b) - remove this property and let OS decide what to do with thermal
zones by default.

On the linkage, there are essentially two other approaches, as I
mentioned below in the original RFC. First would be to have the
thermal_zone binding inside the node of the device requiring thermal
limits, this way the linkage would be more obvious, I think. Other
approach would be to link them by having a property on the sensor node
to the monitored device node, as suggested in other email.

> Thanks,
> Mark.
> 
> [1] https://lkml.org/lkml/2013/7/17/379
> 
>>
>>> 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@...000{ temperature = <100000>; /*
>>> milliCelsius hysteresis = <2000>; /* milliCelsius */ type =
>>> <THERMAL_TRIP_PASSIVE>; }; crit@...000{ 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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