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

[Krzysztof Kozlowski <krzk@kernel.org>]

On 28/03/2022 15:12, Ahmad Fatoum wrote:
> Hello Adam,
> 
> On 28.03.22 15:06, Adam Ford wrote:
>> On Mon, Mar 28, 2022 at 7:56 AM Krzysztof Kozlowski <krzk@kernel.org> 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.
>>>
>>> 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.
>>>
>>>>
>>>> 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.
>>
>> I was under the impression Ahmad didn't want me to add the imx8mm
>> compatible to the DTS, but instead, add the imx8mp compatible into the
>> driver so it binds directly to an imx8mp.

It's not really related the bindings, so don't ask me.

Best regards,
Krzysztof