Re: [PATCH 3/5] arm64: dts: imx8mp: Enable HS400-ES

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Krzysztof,

On 28.03.22 14:56, Krzysztof Kozlowski wrote:
> On 28/03/2022 14:45, Adam Ford wrote:
>> On Mon, Mar 28, 2022 at 6:49 AM Krzysztof Kozlowski <krzk@kernel.org> wrote:
>>>
>>> On 28/03/2022 13:09, Ahmad Fatoum wrote:
>>>> Hello Adam,
>>>>
>>>> On 28.03.22 12:47, Adam Ford wrote:
>>>>> On Mon, Mar 28, 2022 at 2:20 AM Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
>>>>>>
>>>>>> Hello Adam,
>>>>>>
>>>>>> On 27.03.22 14:38, Adam Ford wrote:
>>>>>>> The SDHC controller in the imx8mp has the same controller
>>>>>>> as the imx8mm which supports HS400-ES. Change the compatible
>>>>>>> fallback to imx8mm to enable it.
>>>>>>
>>>>>> I believe that's a shortcoming of the Linux driver, which should explicitly list
>>>>>> fsl,imx8mp-usdhc in its compatibles and enable HS400-ES for it.
>>>>>>
>>>>>> I find dropping compatibles problematic, because like Linux matching
>>>>>> fsl,imx8mm-usdhc, but not fsl,imx8mp-usdhc, other software may match
>>>>>> fsl,imx7d-usdhc, but not fsl,imx8[mp]-usdhc.
>>>>>>
>>>>>> I'd prefer that either the kernel driver gains extra compatibles or that
>>>>>> the DTS lists extra compatibles and we refrain from dropping existing
>>>>>> (correct) ones.
>>>>>>
>>>>>
>>>>> I would argue that imx7d is not correct since the IP blocks between
>>>>> imx7d and imx8mm have different flags/quirks.  One of which includes
>>>>> HS400-ES, but there are other differences as well.
>>>>
>>>> The DTS currently says that an fsl,imx7d-usdhc is a subset of an
>>>> fsl,imx8mm-usdhc. So a driver could treat both HW the exact same
>>>> by focusing on the i.MX7D parts. Linux apparently did exactly
>>>> that so far. Is this not accurate?
>>>>
>>>>
>>>>>> What do you think?
>>>>>
>>>>> From my understanding of the fallback compatibility strings is to
>>>>> avoid having to add more and more compatible strings to the drivers
>>>>> when they do not serve a functional purpose. Based On a conversation
>>>>> with Krzysztof [1], he suggested we update the YAML file based on the
>>>>> fallback, but he wanted NXP to give their feedback as to what the
>>>>> right fallback strings should be.  Haibo from NXP sent me a hierarchy
>>>>> [1] which is what I used to update the YAML file.  Based on the YAML
>>>>> file, the fallback in each DTSI file was updated to ensure the use of
>>>>> the proper IP block.
>>>>
>>>> Myself I am in favor of moving to three compatibles instead of dropping one.
>>>> For some theoretical fsl,imx8mf-usdhc that's supposed to be exactly the same
>>>> as a fsl,imx8mm-usdhc, I don't mind omitting the fsl,imx7d-usdhc compatible,
>>>> but for existing device trees, this may introduce needless potential breakage
>>>> for other software that also uses Linux device trees.
>>>>
>>>
>>> Affecting existing users is indeed a concern with this approach, because
>>> in-kernel DTS might be used in other projects as well.
>>>
>>> I still cannot find here the answer whether fsl,imx8mm-usdhc is actually
>>> compatible with fsl,imx7d-usdhc. It's not about driver, but about
>>> hardware and programming model. imx8mm can support additional features
>>> and still be compatible with imx7d. However if any flags of imx7d are
>>> actually not valid for imx8mm, then it's different case.
>>
>> The imx7d flags are:
>>  ESDHC_FLAG_USDHC
>> ESDHC_FLAG_STD_TUNING
>>  ESDHC_FLAG_HAVE_CAP1
>> ESDHC_FLAG_HS200
>>  ESDHC_FLAG_HS400
>>  ESDHC_FLAG_STATE_LOST_IN_LPMODE
>>  ESDHC_FLAG_BROKEN_AUTO_CMD23,
>>
>> The imx8mm flags are:
>>  ESDHC_FLAG_USDHC
>>  ESDHC_FLAG_STD_TUNING
>>  ESDHC_FLAG_HAVE_CAP1
>> ESDHC_FLAG_HS200
>>  ESDHC_FLAG_HS400
>>  ESDHC_FLAG_HS400_ES
>>  ESDHC_FLAG_STATE_LOST_IN_LPMODE
>>
>> It does not have the ESDHC_FLAG_BROKEN_AUTO_CMD23 that is present in the imx7d.
> 
> AFAIU, it looks imx8mm is compatible with imx7d, because the broken
> acmd23 only limits the features. If imx8mm binds according to imx7d, it
> will not support acmd23 and HS400-ES.
> 
> Having three compatibles is therefore also OK.

My thoughts, exactly.

> You could also add two cases:
> 1. three compatibles, deprecated: True,
> 2. two compatibles, without imx7d.
> 
> Existing DTS stays with three compatibles for two years and later gets
> converted to two compatibles. New DTS should use two compatibles.
> 
> It's quite a lot of churn, but would make in the long term bindings
> correct and also not break other users/projects.

I don't see why we need to deprecate the old binding. New SoCs
can be imx8mm-usdhc compatible from the beginning and need not
care about the old binding. Existing SoCs can just remain imx7d-usdhc
compatible as they are now.

I don't see what the deprecation accomplishes.

>> Maybe Haibo can comment on whether or not that would be an issue for the 8m[mnp]
>>
>> I will defer to Krzysztof and Haibo as to the proper method that we
>> should add HS400-ES.  I don't have an issue adding the imx8mn or
>> imx8mp compatible flags to the esdhc driver if that's the decision.
> 
> I don't get what's the problem with HS400-ES. In any case (your patch
> here, other ideas) your DTS will bind to imx8mm-usdhc which has HS400-ES.

Cheers,
Ahmad

> Best regards,
> Krzysztof


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