PECI API?

[Jae Hyun Yoo]

>>>> On Tue, Oct 31, 2017 at 9:26 AM, Jae Hyun Yoo <jae.hyun.yoo at linux.intel.com> wrote:
>>>>>> On Oct 23, 2017, at 1:03 PM, Jae Hyun Yoo <jae.hyun.yoo at linux.intel.com> wrote:
>>>>>>
>>>>>> Hi Dave,
>>>>>>
>>>>>> I'm currently working on PECI kernel driver
>>>>>
>>>>> I'm curious about the high level structure.  I'm sure others are as well.
>>>>> Anything you can share would be informative and appreciated!
>>>>>
>>>>> A couple questions that popped into my head:
>>>>>
>>>>>  - Would there be a new Linux bus type or core framework for this?
>>>>>  - How many drivers would there be for a full stack.  Something like this?
>>>>>      - client? (hwmon, for example)
>>>>>      - core? (common code)
>>>>>      - bmc specific implementation? (aspeed, nuvoton, emulated
>>>>> differences)
>>>>>  - Have you considered using DT bindings and/or how they would look?
>>>>>
>>>>> These questions are motivated by the recent upstreaming experience 
>>>>> with FSI (flexible support interface) where a similar structure was used.
>>>>> FSI on POWER feels similar to PECI in terms of usage and features 
>>>>> so I thought I'd just throw this out there as a possible reference point to consider.
>>>>
>>>> PECI is using single-wired interface which is different from other 
>>>> popular interfaces such as I2C and MTD, and therefore it doesn't 
>>>> have any common core framework in kernel so I'm adding the PECI 
>>>> main contorl driver as an misc type and the other one into hwmon subsystem.
>>>> The reason why I seperate the implementation into two drivers is, 
>>>> PECI can be used not only for temperature monitoring but also for 
>>>> platform manageability, processor diagnostics and failure analysis, 
>>>> so the misc control driver will be used as a common PECI driver for 
>>>> all those purposes flexibly and the hwmon subsystem driver will use 
>>>> the common PECI driver just for temperature monitoring. These 
>>>> drivers will be BMC specific implementation which support Aspeed 
>>>> shipset only. Support for Nuvoton chipset was not considered in my 
>>>> implementation because Nuvoton has different HW and register 
>>>> scheme, also Nuvoton already has its dedicated driver 
>>>> implementation in hwmon subsystem for their each chipset variant (nct6683.c  nct6775.c  nct7802.c).
>>>>
>>>
>>> Nuvoton is starting to submit support for their Poleg BMC to 
>>> upstream (http://lists.infradead.org/pipermail/linux-arm-kernel/2017-October/538226.html).
>>> This BMC includes a PECI controller similar to the Aspeed design but 
>>> with a different register layout.  At a minimum, the misc driver 
>>> needs to support multiple backend drivers to allow Nuvoton to 
>>> implement the same interface.  The chips you listed that are already 
>>> in hwmon are for Nuvoton's SuperIOs, not their BMCs.
>>>
>>
>> Thanks for your pointing out of the current Poleg BMC upstreaming. I didn't know about that before.
>> Ideally, it would be great if we support all BMC PECI controllers in a 
>> single device driver but we should consider some dependencies such as 
>> SCU register setting in bootloader, clock setting for the PECI controller HW block and etc that would vary on each BMC controller chipset.
>> Usually, these dependencies should be covered by kernel config and device tree settings.
>> My thought is, each BMC controller should have its own PECI misc 
>> driver then we could selectively enable one by kernel configuration.
>>
>
> Are you expecting each BMC controller's PECI misc driver to re-implement the device ioctls?
> If I assume the misc device and ioctl implementation are shared, I can't see how adding a subsystem would be significantly more work.
> Doing so would clarify what the boundaries are between controller implementation and protocol behavior.
>

Okay, I agreed. That is reasonable concern. At least, if possible, we should provide compatible
ioctl set. I'll check its feasibility after getting Nuvoton's datasheet and their SDK.

>>>>>> and hwmon driver implementation. The kernel driver would provide these PECI commands as ioctls:
>>>>>>
>>>>>> - low-level PECI xfer command
>>>>>
>>>>> Would a separate 'dev' driver similar to i2c-dev make sense here?  Just thinking out loud.
>>>>>
>>>>
>>>> Yes, drivers will be seperated into two but it's hard to say that this way is similar to i2c-dev.
>>>> It would have a bit different shape.
>>>>
>>>
>>> I'm not terribly familiar with the PECI protocol.  I'll see about getting a copy of the spec.
>>> From what I can find via searches, it looks like individual nodes on the bus are addressed similar to I2C.
>>> I'd expect that to be similar to how i2c-dev is structured: a 
>>> kobject per master and a kobject per address on the bus.  That way, 
>>> drivers can be bound to individual addresses. The misc driver would focus on exposing interacting with a specific address on the bus in a generic fashion.
>>>
>>
>> As you said, it would be very useful if kernel has core bus framework 
>> like I2C, but current kernel doesn't have the core bus framework for 
>> PECI, and it would be a hugh project itself if we are going to implement one.
>
> Really?  IBM did so for FSI and it really helped with understanding the design.
>

Yes, IBM did really great work on FSI, Kudos to them.

>> Generally, PECI bus topology is very simple unlike I2C. Usually in a 
>> single system, there is only one BMC controller and it has connections 
>> with CPUs, that's all. I don't see an advantage of using core bus framework on this simple interface.
>>
>
> Ideally, an hwmon driver for PECI on an Intel CPU only needs to know how to issue PECI commands to that device.
> What address it is at and how the bus delivers the command to the node are irrelevant details.
> How do you plan to describe the PECI bus in a dts?
> Can I use the same dt bindings for the Intel CPU's PECI interface for both Aspeed and Nuvoton?
>

HW dependent parameters will be added into dts. All SoCs has its own dt binding set so it couldn't
be shared between Aspeed and Nuvoton. 

>>>>>> - Ping()
>>>>>> - GetDIB()
>>>>>> - GetTemp()
>>>>>> - RdPkgConfig()
>>>>>> - WrPkgConfig()
>>>>>> - RdIAMSR()
>>>>>> - RdPCIConfigLocal()
>>>>>> - WrPCIConfigLocal()
>>>>>> - RdPCIConfig()
>>>>>>
>>>>>> Also, through the hwmon driver, these temperature monitoring features would be provided:
>>>>>>
>>>>>> - Core temperature
>>>>>> - DTS thermal margin (hysteresis)
>>>>>> - DDR DIMM temperature
>>>>>> - etc.
>>>>>
>>>>> Sweet!
>>>>>
>>>>>>
>>>>>> Patches will come in to upstream when it is ready.
>>>>>>
>>>>>> Cheers,
>>>>>> Jae
>>>>>
>>>>> For completeness, a port of the Aspeed SDK PECI driver was proposed in 2016 but it didn't go anywhere:
>>>>>
>>>>> https://lists.ozlabs.org/pipermail/openbmc/2016-August/004381.html
>>>>>
>>>>> thx - brad
>>>>>
>>>>
>>>> My implementation is also heavily based on the Aspeed SDK driver 
>>>> but modified a lot to provide more suitable functionality for openbmc project. Hopefully, it could be introduced soon.
>>>>
>>>> thx,
>>>> Jae
>>