On 12 May 2015 at 23:46, Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > Since the regulator used for the SDMMC IO voltage is not expected to > draw a lot of current, most systems will probably use an inexpensive > LDO for it. LDO regulators apparently have the feature that they > don't actively drive the voltage down--they wait for other components > in the system to drag the voltage down. Thus they will transition > faster under heavy loads and slower under light loads. > > During an SDMMC voltage change from 3.3V to 1.8V, we are almost > certainly under a light load. To be specific: > * The regulator is hooked through pulls to CMD0-3 and DAT. Probably > the CMD pulls are something like 47K and the DAT is something like > 10K. > * The card is supposed to be driving DAT0-3 low during voltage change > which will draw _some_ current, but not a lot. > * The regulator is also provided to the SDMMC host controller, but the > SDMMC host controller is in open drain mode during the voltage > change and so shouldn't be drawing much current. > > In order to keep the SDMMC host working properly (or for noise > reasons), there might also be a capacitor attached to the SDMMC IO > regulator. This also will have the effect of slowing down transitions > of the regulator, especially under light loads. > > From experimental evidence, we've seen the voltage change fail if the > card doesn't detect that the voltage fell to less than about 2.3V when > we turn on the clock. On one device (that admittedly had a 47K CMD > pullup instead of a 10K CMD pullup) we saw that the voltage was just > about 2.3V after 5ms and thus the voltage change would sometimes fail. > Doubling the delay gave margin and made the voltage change work 100% > of the time, despite the slightly weaker CMD pull. > > At the moment submitting this as an RFC patch since my problem _could_ > be fixed by increasing the pull strength (or using a smaller > capacitor). However being a little bit more lenient to strange > hardware could also be a good thing. > > Signed-off-by: Doug Anderson <dianders@xxxxxxxxxxxx> Thanks, applied. Kind regards Uffe > --- > drivers/mmc/core/core.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c > index 92e7671..a7e6110 100644 > --- a/drivers/mmc/core/core.c > +++ b/drivers/mmc/core/core.c > @@ -1551,8 +1551,8 @@ int mmc_set_signal_voltage(struct mmc_host *host, int signal_voltage, u32 ocr) > goto power_cycle; > } > > - /* Keep clock gated for at least 5 ms */ > - mmc_delay(5); > + /* Keep clock gated for at least 10 ms, though spec only says 5 ms */ > + mmc_delay(10); > host->ios.clock = clock; > mmc_set_ios(host); > > -- > 2.2.0.rc0.207.ga3a616c > -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
