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Subject: Re: [v4 07/11] dt-bindings: hwmon: Add documents for PECI hwmon
 client drivers
To:     Rob Herring <robh@kernel.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Haiyue Wang <haiyue.wang@linux.intel.com>,
        Vernon Mauery <vernon.mauery@linux.intel.com>,
        James Feist <james.feist@linux.intel.com>,
        devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Jeffery <andrew@aj.id.au>,
        Arnd Bergmann <arnd@arndb.de>,
        Jason M Biils <jason.m.bills@linux.intel.com>,
        Joel Stanley <joel@jms.id.au>
References: <20180521195905.27983-1-jae.hyun.yoo@linux.intel.com>
 <20180522164230.GA11707@rob-hp-laptop>
 <a05b44a2-ecc8-b028-4308-47d88523a6f1@linux.intel.com>
 <CAL_JsqJ8rjRzaqasjbdq-jkYfWxdo72YnmKAVNxDwo6wtyv1xg@mail.gmail.com>
 <51752610-d2c1-7fbc-101e-e99346fa29e4@linux.intel.com>
 <CAL_JsqK9ap=p67HDb_aka809nvpzL3K9VOdbMKJbf3EMR8D2XQ@mail.gmail.com>
From:   Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Message-ID: <22bd0e23-69ad-5858-656e-16c77007913c@linux.intel.com>
Date:   Wed, 23 May 2018 13:03:02 -0700
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On 5/23/2018 12:33 PM, Rob Herring wrote:
> On Wed, May 23, 2018 at 11:37 AM, Jae Hyun Yoo
> <jae.hyun.yoo@linux.intel.com> wrote:
>> On 5/23/2018 8:11 AM, Rob Herring wrote:
>>>
>>> On Tue, May 22, 2018 at 12:18 PM, Jae Hyun Yoo
>>> <jae.hyun.yoo@linux.intel.com> wrote:
>>>>
>>>> On 5/22/2018 9:42 AM, Rob Herring wrote:
>>>>>
>>>>>
>>>>> On Mon, May 21, 2018 at 12:59:05PM -0700, Jae Hyun Yoo wrote:
>>>>>>
>>>>>>
>>>>>> This commit adds dt-bindings documents for PECI hwmon client drivers.
>>>>>>
>>>>>> Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
>>>>>> Reviewed-by: Haiyue Wang <haiyue.wang@linux.intel.com>
>>>>>> Reviewed-by: James Feist <james.feist@linux.intel.com>
>>>>>> Reviewed-by: Vernon Mauery <vernon.mauery@linux.intel.com>
>>>>>> Cc: Andrew Jeffery <andrew@aj.id.au>
>>>>>> Cc: Arnd Bergmann <arnd@arndb.de>
>>>>>> Cc: Jason M Biils <jason.m.bills@linux.intel.com>
>>>>>> Cc: Joel Stanley <joel@jms.id.au>
>>>>>> ---
>>>>>>     .../bindings/hwmon/peci-cputemp.txt           | 23
>>>>>> ++++++++++++++++++
>>>>>>     .../bindings/hwmon/peci-dimmtemp.txt          | 24
>>>>>> +++++++++++++++++++
>>>>>>     2 files changed, 47 insertions(+)
>>>>>>     create mode 100644
>>>>>> Documentation/devicetree/bindings/hwmon/peci-cputemp.txt
>>>>>>     create mode 100644
>>>>>> Documentation/devicetree/bindings/hwmon/peci-dimmtemp.txt
>>>>>>
>>>>>> diff --git a/Documentation/devicetree/bindings/hwmon/peci-cputemp.txt
>>>>>> b/Documentation/devicetree/bindings/hwmon/peci-cputemp.txt
>>>>>> new file mode 100644
>>>>>> index 000000000000..2f59aee12d9e
>>>>>> --- /dev/null
>>>>>> +++ b/Documentation/devicetree/bindings/hwmon/peci-cputemp.txt
>>>>>> @@ -0,0 +1,23 @@
>>>>>> +Bindings for Intel PECI (Platform Environment Control Interface)
>>>>>> cputemp
>>>>>> driver.
>>>>>> +
>>>>>> +Required properties:
>>>>>> +- compatible : Should be "intel,peci-cputemp".
>>>>>> +- reg        : Should contain address of a client CPU. Address range
>>>>>> of
>>>>>> CPU
>>>>>> +              clients is starting from 0x30 based on PECI
>>>>>> specification.
>>>>>> +
>>>>>> +Example:
>>>>>> +       peci-bus@0 {
>>>>>> +               #address-cells = <1>;
>>>>>> +               #size-cells = <0>;
>>>>>> +               < more properties >
>>>>>> +
>>>>>> +               peci-cputemp@30 {
>>>>>> +                       compatible = "intel,peci-cputemp";
>>>>>> +                       reg = <0x30>;
>>>>>> +               };
>>>>>
>>>>>
>>>>>
>>>>> [...]
>>>>>
>>>>>> +               peci-dimmtemp@30 {
>>>>>> +                       compatible = "intel,peci-dimmtemp";
>>>>>> +                       reg = <0x30>;
>>>>>> +               };
>>>>>
>>>>>
>>>>>
>>>>> As I said in the prior version, 2 nodes at the same address is wrong.
>>>>>
>>>>> Rob
>>>>>
>>>>
>>>> In PECI bus, there is one and only bus host (adapter) and multiple
>>>> clients on a PECI bus, and PECI spec doesn't allow multiple originators
>>>> so only the host device can originate message.
>>>
>>>
>>> Yes, I get that. A single host still has to address slave devices.
>>>
>>>> In this implementation,
>>>> all message transactions on a bus from client driver modules and user
>>>> space will be serialized well in the PECI core bus driver so bus
>>>> occupation and traffic arbitration will be managed well in the PECI core
>>>> bus driver even in case of a bus has 2 client drivers at the same
>>>> address. I'm sure that this implementation doesn't make that kind of
>>>> problem to OS.
>>>
>>>
>>> Multiple clients to a single device is common, but that is a software
>>> problem and doesn't belong in DT.
>>>
>>> I don't think there is a single other case in the kernel where
>>> multiple drivers can bind to the same device at a given bus address.
>>> That is why we have things like MFD. Though in this case, why can't
>>> one hwmon driver register multiple hwmon devices (cpu and dimm temps)?
>>>
>>
>> It was implemented as a single driver until v2 but dimm temps need
>> delayed creation unlikely the cpu temps on hwmon subsystem because of
>> memory training behavior of remote x86 cpus. Since hwmon doesn't allow
>> incremental creation, I had to divide it into two, cputemp and dimmtemp,
>> so that cputemp can be registered immediately when the remote x86 cpu
>> turns on and dimmtemp can be registered by delayed creation. It is the
>> reason why I had to make the two hwmon driver modules that sharing a
>> single device address.
> 
> That all sounds like kernel problems to me. Stop designing your DT
> binding around what the kernel can or can't *currently* support.
> 
>> Additionally, PECI isn't limited for temperature
>> monitoring feature but it can be used for other functions such as
>> platform management, cpu interface tuning and diagnostics and failure
>> analysis, so in case of adding a new driver for the functions, we should
>> add an another DT node which is sharing the same cpu address.
> 
> No, the driver should add support for those additional functions.
> Perhaps you will need to use MFD.
> 

Do you mean that the device address sharing is acceptable if I make
these nodes under "simple-mfd"?

Thanks,

-Jae