[PATCH RFC 1/3] DT: bindings: mmc: Add property for 3.3V only support

[Ulf Hansson]

On 18 August 2016 at 14:25, Adrian Hunter <adrian.hunter at intel.com> wrote:
> On 11/08/16 03:48, Shawn Lin wrote:
>> + Adrian
>>
>> Let's queue Adrian here who now maintains SDHCI stuff.
>
> SDHCI drivers may not implement no-1-8-v in a consistent manner, but as far
> as I can see, the meaning is still clear: 1.8V will not be used for either
> supply or signaling.

Okay.

>
> SDHCI is complicated because the SDHCI specification does not cover eMMC.
> From the perspective of SDHCI, the only 1.8V modes are the UHS-I modes, so
> support for 1.8V signaling is the same as support for one of those modes
> (the spec even says as much).  But what happens is that the host controller
> can support those modes but the board can't supply 1.8V so the drivers
> remove capability for the modes.  Support for 1.8V supply has a capability
> bit which drivers can override if necessary but removable SD cards don't
> support 1.8V supply anyway, so the issue doesn't arise if the host
> controller is only used for uSD cards.

By looking how SDHCI uses the SDHCI_SUPPORT_DDR50 in conjunction with
SDHCI_QUIRK2_NO_1_8_V (which is set when no-1-8-v DT property is
provided), this becomes a bit messy.

>From Adrian's summary above, it then seems appropriate to limit the
no-1-8-v DT property to apply only to capabilities related to SD
cards, as I assume that also was the original purpose.

Do you think it's possible to clean up this in sdhci when assigning
the caps masks, and then also clarify the no-1-8-v DT binding in the
documentation?

Regarding the new DT binding proposed to be added, mmc-ddr-3_3v, it
seems we need this to be able to properly describe the HW.
Rob, do you have an issue with adding this binding? I am thinking that
we already have mmc-ddr-1_8v and mmc-ddr-1_2v, so it just follow
existing pattern.

[...]

Kind regards
Uffe