[PATCH 1/2] mmc: Imply HS200 cap with mmc-hs400 prop to match linux

[Jonas Karlman]

Hi Quentin,

On 2024-04-09 17:27, Quentin Schulz wrote:
> Hi Jonas,
> 
> On 4/8/24 23:06, Jonas Karlman wrote:
>> eMMC nodes in linux device tree files typically only contain a mmc-hs400
>> prop to signal support for both HS400 and HS200. However, U-Boot require
>> an explicit mmc-hs200 prop to signal support for the HS200 mode.
>>  > Fix this by follow linux and imply HS200 cap when HS400 cap is signaled
>> using a mmc-hs400 prop.
>>
> 
> Technically speaking, the DT binding should be the one and only source 
> of truth and should be implementation-agnostic.
> 
> There it says:
> """
>    mmc-hs400-1_2v:
>      $ref: /schemas/types.yaml#/definitions/flag
>      description:
>        eMMC HS400 mode (1.2V I/O) is supported.
> 
>    mmc-hs400-1_8v:
>      $ref: /schemas/types.yaml#/definitions/flag
>      description:
>        eMMC HS400 mode (1.8V I/O) is supported.
> """
> 
> So I'd say, the DTs should be fixed to add mmc-hs200 as well wherever it 
> makes sense.
> 
> The point of the DT/DT binding is to be system-agnostic and 
> representative of the **HW** implementation. At least that's what the DT 
> people want it to be.
> 
> If the eMMC standard doesn't allow to have HS400 without HS200, then I 
> think this change is acceptable as is, because it is the reality of the 
> HW standard. Couldn't find this implied in the standard though (but I 
> just skimmed through).
> 
> It's also quite surprising, as it's not because the eMMC works with 
> HS400 that it necessarily does with HS200 or that it's desired (EMI, 
> signal integrity/stability, etc...)?
> 
> Now, it wouldn't be the first time U-Boot follows whatever is done in 
> Linux, so... up to you/the maintainers :)

Agree that implying HS200 does not fully make sense, however it was part
of the original Linux binding when HS400 was added in v3.16-rc1 [1] so I
think that this is the expected behavior and changing it may be an ABI
breakage.

The original Linux commit [1] mention:
"Specially, if host can support HS400, it means that host can also
support HS200."

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c373eb489b27b268c9b8c267b212d10864bc8cdd

Regards,
Jonas

> 
> Reviewed-by: Quentin Schulz <quentin.schulz at theobrma-systems.com>
> 
> Cheers,
> Quentin