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

From: Krzysztof Kozlowski
Date: Mon Aug 08 2022 - 02:23:22 EST


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@xxxxxxx> 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@xxxxxxxxx>
>>>> ---
>>>> 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