IMX6 USDHC card detection

[Russell King - ARM Linux]

On Sat, Oct 05, 2013 at 05:20:01PM +0800, Shawn Guo wrote:
> DATA3 as Card Detection Pin:
> If this bit is set, DATA3 should be pulled down to act as a card
> detection pin. Be cautious when using this feature, because DATA3 is
> also a chip-select for the SPI mode. A pull-down on this pin and CMD0
> may set the card into the SPI mode, which the uSDHC does not support.

The issue there is if the pull-down is too strong.

DATA3 is pulled up by the SD card via a 50k internal resistor.  If the
IMX6 is programmed to pull down via 100k, that will result in 2/3s of
the card supply on DATA3.  That should be fine given the card thresholds:

				Min		Max
Input High Voltage	VIH	0.625*VDD	VDD+0.3		V
Input Low Voltage	VIL	VSS-0.3		0.25*VDD	V

Of course, if a board has an additional external pull-down resistor on
DAT3, that will lower the DATA3 voltage level, so that's where a problem
could be.

Would you be happy if I added new pinctrl entries to imx6qdl.dtsi which
pulled down the DATA3 line - that way, existing DT setups will not be
affected, and they can switch over if they wish to using controller
based card detection.

Thanks.