[PATCH] mmc: dw_mmc: try pick the exact same voltage as vmmc for vqmmc

[Doug Anderson]

Ulf,

On Mon, Nov 24, 2014 at 5:29 AM, Ulf Hansson <ulf.hansson at linaro.org> wrote:
>> 2. Several people I've talked to have expressed concerns that our
>> minimum value is 2.7V.  Apparently that's really on the edge and makes
>> EEs a little nervous.  The quick sample of cards sitting on my desk
>> shows that they seem to claim 0x00ff8000, which doesn't include 2.7V.
>
> 0x00ff8000 states what values of VDD levels the device supports. Not VIO.

Yup, I was just pointing out that possibly others were trying to get a
little bit of margin (not going all the way down to 2.7V) too.

>> Both of the above make me feel like dw_mmc should try its best to pick
>> a value for vqmmc that is closest to the value of vmmc (and >= 2.7V).
>> That also happens to make us work exactly like hosts where vmmc and
>> vqmmc are supplied by the same supply.
>
> I do see your point. And I agree that it would be nice to achieve
> something like this.
>
> The question is how to do this. For sure, we need to involve the mmc
> core to handle this correctly.

You could add a function to the core that we could call from
dw_mci_switch_voltage() and it would do all the work except trying to
set the UHS register.  That would certainly make it easy.  That could
try to set the highest voltage that is <= vmmc when we're at
MMC_SIGNAL_VOLTAGE_330.

-Doug