Re: [PATCH 4/5 v3] mmc: add a function to get a regulator, supplying card's Vdd

[Ulf Hansson <ulf.hansson@stericsson.com>]

On 06/11/2012 04:36 PM, Philip Rakity wrote:
>
> On Jun 11, 2012, at 7:23 AM, Mark Brown wrote:
>
>> On Mon, Jun 11, 2012 at 02:39:29PM +0200, Guennadi Liakhovetski wrote:
>>> On Mon, 28 May 2012, Mark Brown wrote:
>>
>>>> Even if the supply is always on you can easily provide an always on
>>>> regulator.  From the point of view of the consumer if there are two
>>>> physical supplies needed it's much clearer and easier to just code that
>>>> and deal with the fact that we might have no control over them
>>>> externally, there's nothing device specific about that process.
>>
>>> Mark, I'm not sure I understand your above comment correctly. The
>>> function, that I add in this patch doesn't do much now, that we removed
>>
>>> IIUC, to satisfy Philip's requirement for a second regulator it would
>>> suffice to just issue one more devm_regulator_get() and let the caller
>>> deal with any errors. Is this what you're suggesting or are you against
>>> adding the second regulator? We could of course also add it later, but
>>> that would change the function prototype (unless we add regulator pointers
>>> to struct mmc_host itself). Whith existing users such changes are a bit
>>> messy...
>>
>> Yes, that's what I'm suggesting.  I guess you could wrap the two
>> regulators up in a small struct without it being too invasive?
>
>
> Some background notes:
> Historically SD cards operated with the same vccq (signaling for dat and clk pins)
> and vcc (core) voltage.
> usually 3.3v -->  we would use one regulator to supply both.  The power (regulator)
> was always on and as far as the SD code was concerned never exposed
> (eg regulator_get (vmmc) returned an error).
>
> Now enter UHS cards.  They allow (require) the vccq voltage change from
> 3.3v to 1.8v when operating in UHS mode (DDR50, SDR50, SDR104).
> They also note that when a voltage switch is done to move the card
> into UHS mode (3.3V to 1.8V) the switch may fail.  In this case vcc
> needs to turned off and then back on to cause the card to reinitialize.
>
> This led me to the conclusion two regulators are required.
> vccq must be switchable between 3.3 and 1.8
> vcc must be able to be turned off/on and may be switchable to a lower voltage.
>
> Note: embedded MMC is different (eMMC).  Since it is embedded it is
> assumed by the spec that the board designed knows that they are doing
> and the correct voltages are supplied.
>

Just some additional input;
In eMMC scenario you have two regulators as well. One called vcc (power 
to the card) and one called vccq (io-voltage). Very much depending on 
the hw board design, you may use one, two or no regulator at all for 
these power(s).

Kind regards
Ulf Hansson