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@xxxxxxxxxxxxxxxx>: 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@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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