Date:   Wed, 18 Apr 2018 16:28:41 -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 07/10] Documentation: dt-bindings: Add documents for
 PECI hwmon client drivers

On Wed, Apr 18, 2018 at 3:28 PM, Jae Hyun Yoo
<jae.hyun.yoo@...ux.intel.com> wrote:
> On 4/18/2018 7:32 AM, Rob Herring wrote:
>>
>> On Tue, Apr 17, 2018 at 3:40 PM, Jae Hyun Yoo
>> <jae.hyun.yoo@...ux.intel.com> wrote:
>>>
>>> On 4/16/2018 4:51 PM, Jae Hyun Yoo wrote:
>>>>
>>>>
>>>> On 4/16/2018 4:22 PM, Jae Hyun Yoo wrote:
>>>>>
>>>>>
>>>>> On 4/16/2018 11:14 AM, Rob Herring wrote:
>>>>>>
>>>>>>
>>>>>> On Tue, Apr 10, 2018 at 11:32:09AM -0700, Jae Hyun Yoo wrote:
>>>>>>>
>>>>>>>
>>>>>>> This commit adds dt-bindings documents for PECI cputemp and dimmtemp
>>>>>>> client
>>>>>>> drivers.
>>>>>>
>>>>>>
>>>>>>
>>>
>>> [...]
>>>
>>>>>>> +Example:
>>>>>>> +    peci-bus@0 {
>>>>>>> +        #address-cells = <1>;
>>>>>>> +        #size-cells = <0>;
>>>>>>> +        < more properties >
>>>>>>> +
>>>>>>> +        peci-dimmtemp@...0 {
>>>>>>
>>>>>>
>>>>>>
>>>>>> unit-address is wrong.
>>>>>>
>>>>>
>>>>> Will fix it using the reg value.
>>>>>
>>>>>> It is a different bus from cputemp? Otherwise, you have conflicting
>>>>>> addresses. If that's the case, probably should make it clear by
>>>>>> showing
>>>>>> different host adapters for each example.
>>>>>>
>>>>>
>>>>> It could be the same bus with cputemp. Also, client address sharing is
>>>>> possible by PECI core if the functionality is different. I mean,
>>>>> cputemp and
>>>>> dimmtemp targeting the same client is possible case like this.
>>>>> peci-cputemp@30
>>>>> peci-dimmtemp@30
>>>>>
>>>>
>>>> Oh, I got your point. Probably, I should change these separate settings
>>>> into one like
>>>>
>>>> peci-client@30 {
>>>>       compatible = "intel,peci-client";
>>>>       reg = <0x30>;
>>>> };
>>>>
>>>> Then cputemp and dimmtemp drivers could refer the same compatible
>>>> string.
>>>> Will rewrite it.
>>>>
>>>
>>> I've checked it again and realized that it should use function based node
>>> name like:
>>>
>>> peci-cputemp@30
>>> peci-dimmtemp@30
>>>
>>> If it use the same string like 'peci-client@30', the drivers cannot be
>>> selectively enabled. The client address sharing way is well handled in
>>> PECI
>>> core and this way would be better for the future implementations of other
>>> PECI functional drivers such as crash dump driver and so on. So I'm going
>>> change the unit-address only.
>>
>>
>> 2 nodes at the same address is wrong (and soon dtc will warn you on
>> this). You have 2 potential options. The first is you need additional
>> address information in the DT if these are in fact 2 independent
>> devices. This could be something like a function number to use
>> something from PCI addressing. From what I found on PECI, it doesn't
>> seem to have anything like that. The 2nd option is you have a single
>> DT node which registers multiple hwmon devices. DT nodes and drivers
>> don't have to be 1-1. Don't design your DT nodes from how you want to
>> partition drivers in some OS.
>>
>> Rob
>>
>
> Please correct me if I'm wrong but I'm still thinking that it is
> possible. Also, I did compile it but dtc doesn't make a warning. Let me
> show an another use case which is similar to this case:

I did say *soon*. It's in dtc repo, but not the kernel copy yet.

> In arch/arm/boot/dts/aspeed-g5.dtsi
> [...]
> lpc_host: lpc-host@80 {
>         compatible = "aspeed,ast2500-lpc-host", "simple-mfd", "syscon";
>         reg = <0x80 0x1e0>;
>         reg-io-width = <4>;
>
>         #address-cells = <1>;
>         #size-cells = <1>;
>         ranges = <0x0 0x80 0x1e0>;
>
>         lpc_ctrl: lpc-ctrl@0 {
>                 compatible = "aspeed,ast2500-lpc-ctrl";
>                 reg = <0x0 0x80>;
>                 clocks = <&syscon ASPEED_CLK_GATE_LCLK>;
>                 status = "disabled";
>         };
>
>         lpc_snoop: lpc-snoop@0 {
>                 compatible = "aspeed,ast2500-lpc-snoop";
>                 reg = <0x0 0x80>;
>                 interrupts = <8>;
>                 status = "disabled";
>         };
> }
> [...]
>
> This is device tree setting for LPC interface and its child nodes.
> LPC interface can be used as a multi-functional interface such as
> snoop 80, KCS, SIO and so on. In this use case, lpc-ctrl@0 and
> lpc-snoop@0 are sharing their address range from their individual
> driver modules and they can be registered quite well through both
> static dt or dynamic dtoverlay. PECI is also a multi-functional
> interface which is similar to the above case, I think.

This case too is poor design and should be fixed as well. Simply put,
you can have 2 devices on a bus at the same address without some sort
of mux or arbitration device in the middle. If you have a device/block
with multiple functions provided to the OS, then it is the OS's
problem to arbitrate access. It is not a DT problem because OS's can
vary in how they handle that both from OS to OS and over time.

Rob

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