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Date:   Thu, 18 Aug 2022 17:04:20 +0300
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Subject: Re: [PATCH 1/3] dt-bings: net: fsl,fec: update compatible item
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To:     Shawn Guo <shawnguo@kernel.org>
Cc:     Wei Fang <wei.fang@nxp.com>, davem@davemloft.net,
        edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
        robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
        s.hauer@pengutronix.de, netdev@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@pengutronix.de, festevam@gmail.com, linux-imx@nxp.com,
        peng.fan@nxp.com, ping.bai@nxp.com, sudeep.holla@arm.com,
        linux-arm-kernel@lists.infradead.org, aisheng.dong@nxp.com
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 <20220818135727.GG149610@dragon>
From:   Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
In-Reply-To: <20220818135727.GG149610@dragon>
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On 18/08/2022 16:57, Shawn Guo wrote:
> On Thu, Aug 18, 2022 at 12:46:33PM +0300, Krzysztof Kozlowski wrote:
>> On 18/08/2022 12:22, Shawn Guo wrote:
>>> On Thu, Aug 18, 2022 at 10:51:02AM +0300, Krzysztof Kozlowski wrote:
>>>> On 18/08/2022 04:33, Shawn Guo wrote:
>>>>> On Mon, Jul 04, 2022 at 11:12:09AM +0200, Krzysztof Kozlowski wrote:
>>>>>>> diff --git a/Documentation/devicetree/bindings/net/fsl,fec.yaml b/Documentation/devicetree/bindings/net/fsl,fec.yaml
>>>>>>> index daa2f79a294f..6642c246951b 100644
>>>>>>> --- a/Documentation/devicetree/bindings/net/fsl,fec.yaml
>>>>>>> +++ b/Documentation/devicetree/bindings/net/fsl,fec.yaml
>>>>>>> @@ -40,6 +40,10 @@ properties:
>>>>>>>            - enum:
>>>>>>>                - fsl,imx7d-fec
>>>>>>>            - const: fsl,imx6sx-fec
>>>>>>> +      - items:
>>>>>>> +          - enum:
>>>>>>> +              - fsl,imx8ulp-fec
>>>>>>> +          - const: fsl,imx6ul-fec
>>>>>>
>>>>>> This is wrong.  fsl,imx6ul-fec has to be followed by fsl,imx6q-fec. I
>>>>>> think someone made similar mistakes earlier so this is a mess.
>>>>>
>>>>> Hmm, not sure I follow this.  Supposing we want to have the following
>>>>> compatible for i.MX8ULP FEC, why do we have to have "fsl,imx6q-fec"
>>>>> here?
>>>>>
>>>>> 	fec: ethernet@29950000 {
>>>>> 		compatible = "fsl,imx8ulp-fec", "fsl,imx6ul-fec";
>>>>> 		...
>>>>> 	};
>>>>
>>>> Because a bit earlier this bindings is saying that fsl,imx6ul-fec must
>>>> be followed by fsl,imx6q-fec.
>>>
>>> The FEC driver OF match table suggests that fsl,imx6ul-fec and fsl,imx6q-fec
>>> are not really compatible.
>>>
>>> static const struct of_device_id fec_dt_ids[] = {
>>>         { .compatible = "fsl,imx25-fec", .data = &fec_devtype[IMX25_FEC], },
>>>         { .compatible = "fsl,imx27-fec", .data = &fec_devtype[IMX27_FEC], },
>>>         { .compatible = "fsl,imx28-fec", .data = &fec_devtype[IMX28_FEC], },
>>>         { .compatible = "fsl,imx6q-fec", .data = &fec_devtype[IMX6Q_FEC], },
>>>         { .compatible = "fsl,mvf600-fec", .data = &fec_devtype[MVF600_FEC], },
>>>         { .compatible = "fsl,imx6sx-fec", .data = &fec_devtype[IMX6SX_FEC], },
>>>         { .compatible = "fsl,imx6ul-fec", .data = &fec_devtype[IMX6UL_FEC], },
>>
>> I don't see here any incompatibility. Binding driver with different
>> driver data is not a proof of incompatible devices.
> 
> To me, different driver data is a good sign of incompatibility.  It
> mostly means that software needs to program the hardware block
> differently.

Any device being 100% compatible with old one and having additional
features will have different driver data... so no, it's not a proof.
There are many of such examples and we call them compatible, because the
device could operate when bound by the fallback compatible.

If this is the case here - how do I know? I raised and the answer was
affirmative...

> 
> 
>> Additionally, the
>> binding describes the hardware, not the driver.
>>
>>>         { .compatible = "fsl,imx8mq-fec", .data = &fec_devtype[IMX8MQ_FEC], },
>>>         { .compatible = "fsl,imx8qm-fec", .data = &fec_devtype[IMX8QM_FEC], },
>>>         { /* sentinel */ }
>>> };
>>> MODULE_DEVICE_TABLE(of, fec_dt_ids);
>>>
>>> Should we fix the binding doc?
>>
>> Maybe, I don't know. The binding describes the hardware, so based on it
>> the devices are compatible. Changing this, except ABI impact, would be
>> possible with proper reason, but not based on Linux driver code.
> 
> Well, if Linux driver code is written in the way that hardware requires,
> I guess that's just based on hardware characteristics.
> 
> To me, having a device compatible to two devices that require different
> programming model is unnecessary and confusing.

It's the first time anyone mentions here the programming model is
different... If it is different, the devices are likely not compatible.

However when I raised this issue last time, there were no concerns with
calling them all compatible. Therefore I wonder if the folks working on
this driver actually know what's there... I don't know, I gave
recommendations based on what is described in the binding and expect the
engineer to come with that knowledge.


Best regards,
Krzysztof