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

[Quentin Schulz]

Hi Dragan,

On 4/9/24 21:28, Dragan Simic wrote:

[...]

> Let's keep in mind that the troublesome DT properties describe the
> capabilities of the MMC controller and the board, not the capabilities
> of the MMC storage device.  As we know, eMMC devices provide automatic
> detection capabilities, to allow the host to determine its supported
> modes, and match them with the ones the host is configured to support.
> It's all described in the JEDEC standards.
> 

So why do we have those properties specified in board DTSes instead of 
in the SoC DTSI? Logic would want us to have this defined in one place 
only. I assume the issue is that even if the eMMC chip itself says it 
supports HS400 but the HW routing or some other issue make it impossible 
to use, we need a way to disable it from the DT for that board?

> Having that in mind, I find the approach in the Linux kernel rather
> reasonable, because I highly doubt that some MMC controllers support,
> for example, HS400 without supporting DDR52, or HS400 without supporting
> DDR52.  A reasonable approach for an MMC IP block is to make it capable
> of supporting all the speeds below its highest supported speed, to make
> itself capable of supporting more, if not all, MMC storage devices.
> 

That's true for the IP block which is self-contained in the SoC, but 
it's forgetting about the other part, the eMMC chip/card. It depends on 
the HW routing, where mistakes/limitations can happen. And I don't think 
we have a mechanism today to disable the modes set in the MMC controller 
for a given MMC card from DT (aside from /delete-property/ in board files).

> In fact, I'll probably go ahead and submit a Linux kernel patch that
> updates the descriptions in the binding, to hopefully eliminate any
> ambiguities like these.  I hope you agree.

I for sure do not have enough knowledge in MMC to argue more than I just 
did, so having people with more experience/knowledge have a look at this 
would make sense, let's see what they have to say :)

Cheers,
Quentin