mmc: detailed definition of CD and WP MMC line polarities in DT

Clarify ways to specify write-protect and card-detect MMC lines in FDT.
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>

Documentation/devicetree/bindings/mmc/mmc.txt

@@ -17,12 +17,31 @@ Optional properties:
 - bus-width: Number of data lines, can be <1>, <4>, or <8>.  The default
   will be <1> if the property is absent.
 - wp-gpios: Specify GPIOs for write protection, see gpio binding
-- cd-inverted: when present, polarity on the cd gpio line is inverted
-- wp-inverted: when present, polarity on the wp gpio line is inverted
+- cd-inverted: when present, polarity on the CD line is inverted. See the note
+  below for the case, when a GPIO is used for the CD line
+- wp-inverted: when present, polarity on the WP line is inverted. See the note
+  below for the case, when a GPIO is used for the WP line
 - max-frequency: maximum operating clock frequency
 - no-1-8-v: when present, denotes that 1.8v card voltage is not supported on
   this system, even if the controller claims it is.
 
+*NOTE* on CD and WP polarity. To use common for all SD/MMC host controllers line
+polarity properties, we have to fix the meaning of the "normal" and "inverted"
+line levels. We choose to follow the SDHCI standard, which specifies both those
+lines as "active low." Therefore, using the "cd-inverted" property means, that
+the CD line is active high, i.e. it is high, when a card is inserted. Similar
+logic applies to the "wp-inverted" property.
+
+CD and WP lines can be implemented on the hardware in one of two ways: as GPIOs,
+specified in cd-gpios and wp-gpios properties, or as dedicated pins. Polarity of
+dedicated pins can be specified, using *-inverted properties. GPIO polarity can
+also be specified using the OF_GPIO_ACTIVE_LOW flag. This creates an ambiguity
+in the latter case. We choose to use the XOR logic for GPIO CD and WP lines.
+This means, the two properties are "superimposed," for example leaving the
+OF_GPIO_ACTIVE_LOW flag clear and specifying the respective *-inverted
+property results in a double-inversion and actually means the "normal" line
+polarity is in effect.
+
 Optional SDIO properties:
 - keep-power-in-suspend: Preserves card power during a suspend/resume cycle
 - enable-sdio-wakeup: Enables wake up of host system on SDIO IRQ assertion