Clarify ways to specify write-protect and card-detect MMC lines in FDT. Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@xxxxxx> --- v4: remove Linux-specific references. Documentation/devicetree/bindings/mmc/mmc.txt | 23 +++++++++++++++++++++-- 1 files changed, 21 insertions(+), 2 deletions(-) diff --git a/Documentation/devicetree/bindings/mmc/mmc.txt b/Documentation/devicetree/bindings/mmc/mmc.txt index 34f28ed..24c8552 100644 --- a/Documentation/devicetree/bindings/mmc/mmc.txt +++ b/Documentation/devicetree/bindings/mmc/mmc.txt @@ -18,12 +18,31 @@ Only one of the properties in this section should be supplied: Optional properties: - 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 -- 1.7.2.5 -- 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
