linux-kernel - Re: [v4 07/11] dt-bindings: hwmon: Add documents for PECI hwmon client drivers

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Date:   Wed, 23 May 2018 14:33:24 -0500
From:   Rob Herring <robh@...nel.org>
To:     Jae Hyun Yoo <jae.hyun.yoo@...ux.intel.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Haiyue Wang <haiyue.wang@...ux.intel.com>,
        Vernon Mauery <vernon.mauery@...ux.intel.com>,
        James Feist <james.feist@...ux.intel.com>,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Jeffery <andrew@...id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Jason M Biils <jason.m.bills@...ux.intel.com>,
        Joel Stanley <joel@....id.au>
Subject: Re: [v4 07/11] dt-bindings: hwmon: Add documents for PECI hwmon
 client drivers

On Wed, May 23, 2018 at 11:37 AM, Jae Hyun Yoo
<jae.hyun.yoo@...ux.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@...ux.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@...ux.intel.com>
>>>>> Reviewed-by: Haiyue Wang <haiyue.wang@...ux.intel.com>
>>>>> Reviewed-by: James Feist <james.feist@...ux.intel.com>
>>>>> Reviewed-by: Vernon Mauery <vernon.mauery@...ux.intel.com>
>>>>> Cc: Andrew Jeffery <andrew@...id.au>
>>>>> Cc: Arnd Bergmann <arnd@...db.de>
>>>>> Cc: Jason M Biils <jason.m.bills@...ux.intel.com>
>>>>> Cc: Joel Stanley <joel@....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.

Rob