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

[Ulf Hansson]

On 21 November 2014 at 22:04, Doug Anderson <dianders at chromium.org> wrote:
> Hi,
>
> On Fri, Nov 21, 2014 at 9:42 AM, Doug Anderson <dianders at chromium.org> wrote:
>> Ulf,
>>
>> On Fri, Nov 21, 2014 at 4:06 AM, Ulf Hansson <ulf.hansson at linaro.org> wrote:
>>> [...]
>>>
>>>> Sure
>>>> If the first card is sd2.0 since startup, dw_mci_switch_voltage will not be called,
>>>
>>> That can't be right. mmc_power_up() should trigger
>>> dw_mci_switch_voltage() to be invoked.
>>
>> Hmmm, I think you're right.  Addy: can you double check if it's only
>> the 2nd card for you?  I was thinking that if a regulator is currently
>> 3.3V and you request 2.7 - 3.3V the regulator framework will treat
>> that as a noop.  ...but that definitely doesn't appear to be the case.
>> When I boot up the first time even with no SD card plugged in, I see
>> this at bootup:
>>
>> [    3.042234] vccio_sd: 1800 <--> 3300 mV at 3300 mV
>>
>> ...showing that it started at 3.3V.  Then I see:
>>
>> $ grep "" /sys/class/regulator/regulator.16/{name,microvolts}
>> /sys/class/regulator/regulator.16/name:vccio_sd
>> /sys/class/regulator/regulator.16/microvolts:2700000
>>
>> ...so it is certainly getting changed even with no card plugged in.
>>
>>
>> BTW: I don't actually have one of these failing cards--all of mine
>> work.  Addy, do you know the make and model of the card you have that
>> fails?
>
> Just as a bit of a followup, I did some more digging...
>
> 1. It looks as if we now have a bit of "opposite" logic for vmmc vs.
> vqmmc.  In mmc_power_up() I see that it sets the initial voltage as:
>
>   host->ios.vdd = fls(ocr) - 1;

That's because we would like to supports as many cards as possible.
The policy is based upon that some cards may not support lower
voltages, but most will support higher.

>
> That actually means that we're going to pick the maximum voltage for
> vmmc (of the supported voltages).  For vqmmc dw_mmc is using the
> regulator framework which (as described in my previous message) will
> pick the minimum.

Correct. I have thought this has been inside spec and choosing the
lower value would be preferred to lower power consumption. Maybe we
needs to re-visit this one more time.

Here are some of the interesting sections in the eMMC spec:
10.3.3 Power supply Voltages
"The VCCQ must be defined at equal to or less than VCC".

10.5 Bus signal levels
Push-pull mode:
Voh = 0.75 * VCCQ. (Do note, its VCCQ not VCC).

Summary eMMC: VCCQ must be less and VCC, we should be inside spec.

>From SD spec:
6.6.1 Threshold Level for High Voltage Range
Voh = 0.75 * VDD.

In worst case scenario, VDD = 3.6V and VIO = 2.7V. That gives as the
factor of 0.75, thus we are inside spec but without margins.

>
> 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.

>
>
> 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.

Kind regards
Uffe