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

[Quentin Schulz]

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 :)

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

Cheers,
Quentin