linux-kernel - Re: [PATCH v2] dt-binding: ipmi: add fallback to npcm845 compatible

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Date:   Mon, 8 Aug 2022 09:23:13 +0300
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     minyard@....org, Tomer Maimon <tmaimon77@...il.com>
Cc:     Avi Fishman <avifishman70@...il.com>,
        Tali Perry <tali.perry1@...il.com>,
        Joel Stanley <joel@....id.au>,
        Patrick Venture <venture@...gle.com>,
        Nancy Yuen <yuenn@...gle.com>,
        Benjamin Fair <benjaminfair@...gle.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        openipmi-developer@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2] dt-binding: ipmi: add fallback to npcm845 compatible

On 07/08/2022 14:11, Corey Minyard wrote:
> On Sun, Aug 07, 2022 at 11:03:56AM +0300, Tomer Maimon wrote:
>> Hi Corey,
>>
>> Thanks for your comment.
>>
>> On Fri, 5 Aug 2022 at 14:58, Corey Minyard <minyard@....org> wrote:
>>>
>>> On Thu, Aug 04, 2022 at 09:18:00PM +0300, Tomer Maimon wrote:
>>>> Add to npcm845 KCS compatible string a fallback to npcm750 KCS compatible
>>>> string becuase NPCM845 and NPCM750 BMCs are using identical KCS modules.
>>>>
>>>> Signed-off-by: Tomer Maimon <tmaimon77@...il.com>
>>>> ---
>>>>  Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt b/Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt
>>>> index cbc10a68ddef..4fda76e63396 100644
>>>> --- a/Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt
>>>> +++ b/Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt
>>>> @@ -7,7 +7,7 @@ used to perform in-band IPMI communication with their host.
>>>>  Required properties:
>>>>  - compatible : should be one of
>>>>      "nuvoton,npcm750-kcs-bmc"
>>>> -    "nuvoton,npcm845-kcs-bmc"
>>>> +    "nuvoton,npcm845-kcs-bmc", "nuvoton,npcm750-kcs-bmc"
>>>
>>> This is just wrong.  The compatible is supposed to identify the device,
>>> not the board the device is on.  I think compatible here should be
>>> "npcm7xx-kcs-bmc", and just use that everywhere.  It's fine if that is
>>> used on a board named npcm845.
>> The NPCM8XX is not a board, The Nuvoton NPCM8XX is a fourth-generation
>> BMC SoC device family.
> 
> Ok, but same principle applies.
> 
> If the device is exactly the same, then you would only use one of the
> "npcm7xx-kcs-bmc" and put that in both device trees.  You can use
> "nuvoton,npcm750-kcs-bmc", it's really not that important.  Or even
> "nuvoton,npcm-kcs-bmc"

No, because it is too generic. Compatibles must be specific.

> 
> If the device has a minor difference that can be expressed in a 
> parameter, then create a parameter for it.
> 
> If the device has enough differences that a parameter or two doesn't
> cover it, then you put either nuvoton,npcm750-kcs-bmc or
> nuvoton,npcm750-kcs-bmc in the device tree.  Not both.  Then you need
> two entries in the of_device_id array and you use the data field or
> something to express the difference.

It's quite common to have generic and specific compatibles for
compatible devices and a driver which can match to both of them. I don't
understand where is exactly the problem here?

> 
> Since there appears to be no difference, just put
> "nuvoton,npcm750-kcs-bmc" in the npcm845 and I will drop the patch
> adding all this.  T

Again no, because recommended (also writing bindings document) is always
to have a specific compatible.

> hen a patch can be added saying it applies to both
> the 7xx and 8xx series of BMC SOCs.  If you want to change the name,
> then a patch will be needed for that, but then you will need multiple
> entries in your device tree, but you would not document it as such, as
> there would only be one that applies for this kernel.
> 
> I'm pretty sure the only reason to have muliple compatible entries in a
> device tree is to cover multiple kernels where the name changed.

Best regards,
Krzysztof