linux-kernel - Re: [PATCH v3 04/10] Documentations: dt-bindings: Add a document of PECI adapter driver for Aspeed AST24xx/25xx SoCs

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Date:   Wed, 18 Apr 2018 08:59:58 -0500
From:   Rob Herring <robh@...nel.org>
To:     Jae Hyun Yoo <jae.hyun.yoo@...ux.intel.com>
Cc:     Alan Cox <alan@...ux.intel.com>, Andrew Jeffery <andrew@...id.au>,
        Andrew Lunn <andrew@...n.ch>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Haiyue Wang <haiyue.wang@...ux.intel.com>,
        James Feist <james.feist@...ux.intel.com>,
        Jason M Biils <jason.m.bills@...ux.intel.com>,
        Jean Delvare <jdelvare@...e.com>,
        Joel Stanley <joel@....id.au>,
        Julia Cartwright <juliac@....teric.us>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Milton Miller II <miltonm@...ibm.com>,
        Pavel Machek <pavel@....cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stef van Os <stef.van.os@...drive-technologies.com>,
        Sumeet R Pawnikar <sumeet.r.pawnikar@...el.com>,
        Vernon Mauery <vernon.mauery@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, devicetree@...r.kernel.org,
        Linux HWMON List <linux-hwmon@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>
Subject: Re: [PATCH v3 04/10] Documentations: dt-bindings: Add a document of
 PECI adapter driver for Aspeed AST24xx/25xx SoCs

On Tue, Apr 17, 2018 at 5:06 PM, Jae Hyun Yoo
<jae.hyun.yoo@...ux.intel.com> wrote:
> On 4/17/2018 11:16 AM, Jae Hyun Yoo wrote:
>>
>> On 4/17/2018 6:16 AM, Rob Herring wrote:
>>>
>>> On Mon, Apr 16, 2018 at 6:12 PM, Jae Hyun Yoo
>>> <jae.hyun.yoo@...ux.intel.com> wrote:
>>>>
>>>> On 4/16/2018 11:10 AM, Rob Herring wrote:
>>>>>
>>>>>
>>>>> On Tue, Apr 10, 2018 at 11:32:06AM -0700, Jae Hyun Yoo wrote:
>>>>>>
>>>>>>
>>>>>> This commit adds a dt-bindings document of PECI adapter driver for
>>>>>> Aspeed
>>>>>> AST24xx/25xx SoCs.
>>>
>>>
>>> [...]
>>>
>>>>>> +- clocks            : Should contain clock source for PECI
>>>>>> controller.
>>>>>> +                     Should reference clkin.
>>>>>> +- clock_frequency   : Should contain the operation frequency of PECI
>>>>>> controller
>>>>>> +                     in units of Hz.
>>>>>> +                     187500 ~ 24000000
>>>>>
>>>>>
>>>>>
>>>>> This is the frequency of the bus or used to derive it? It would be
>>>>> better to specify the bus frequency instead and have the driver
>>>>> calculate its internal freq. And then use "bus-frequency" instead.
>>>>>
>>>>
>>>> I agree with you. Actually, it is being used for operation frequency
>>>> setting
>>>> of PECI controller module in SoC so it's different from the meaning of
>>>> "bus-frequency". I'll change it to "operation-frequency".
>>>
>>>
>>> No, now you've gone from a standard property name to something custom.
>>> Why do you need to set the frequency in DT if it is not related to the
>>> interface frequency?
>>>
>>> Rob
>>>
>>
>> Actually, the interface frequency is affected by the operation frequency
>> but there is no description of its relationship in datasheet. I'll check
>> again about the detail to ASPEED chip vendor and will use
>> 'bus-frequency' if available.
>>
>
> I investigated it more deeply. Basically, by the spec, PECI bus speed
> cannot be set as a fixed speed. A PECI bus can have a wide speed range
> from 2Kbps to 2Mbps which is dynamically set by a handshaking sequence
> between an originator and clients called 'timing negotiation' in spec.
> This timing negotiation behavior happens on every single transaction so the
> bus speed also can vary on every transactions. So I'm thinking a custom
> property name for it, 'peci-clk-frequency' if it is acceptable.

Okay, seems bus-frequency is not appropriate here. So use
'clock-frequency' (note the '-' not '_' as that is the standard
property).

Rob