Re: [PATCH v4 2/4] mmc: core: Add mmc_regulator_set_vqmmc()

[Doug Anderson <dianders@chromium.org>]

Ulf,

On Tue, Mar 17, 2015 at 3:23 AM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
>> This will get us within .3V of whatever vmmc is.  If vmmc is 3.3V, it
>> will allow vqmmc of 3.0V - 3.6V.
>>
>> This _seems_ sane to me and given any sane system design we should be
>> fine here, I think.  I can't see someone designing a system where
>> vqmmc was not within .3V of vmmc, can you?  If we think someone will
>> actually build a system where vmmc is 3.3V and vqmmc can't go higher
>> than 2.7V then we'll either need to increase the tolerance here or add
>> a new asymmetric system call like my original patches did.
>
> I know about SoC that supports 3.4V vmmc and 2.9V vqmmc.
>
> What I think we need is the option to have a policy here. We need to
> allow voltage levels stated by the spec and at the same time try chose
> the one best suited. That's not being accomplished here.
>
> Moreover, I wonder whether it's okay (from spec perspective) to have
> vqmmc at a higher voltage level than vmmc. I don't think that's
> allowed, but I might be wrong.

OK, so sounds like I need to add a regulator_set_voltage_tol2()
function that takes in an upper tolerance and a lower.  We can use the
same rough implementation in the core we have today (if Mark is OK
with that) with regulator_set_voltage_tol() but just allow it to be
asymmetric.

>From what I see in the spec for 3.3V cards are supposed to react to a
high signal that is .625 * VDD - VDD + .3


I might not be able to get to this till next week, though...

-Doug