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author | Guennadi Liakhovetski <g.liakhovetski@gmx.de> | 2013-02-15 16:13:51 +0100 |
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committer | Chris Ball <cjb@laptop.org> | 2013-02-24 14:37:19 -0500 |
commit | 6da15e96fb14e7428ecdc69306d8a3287296a968 (patch) | |
tree | 2b6f3f0b6a9a5307a4107f8c37460649165e0fc0 | |
parent | 7b952137fdc5e2c52c990e7cbaf09450bcf01d5e (diff) | |
download | op-kernel-dev-6da15e96fb14e7428ecdc69306d8a3287296a968.zip op-kernel-dev-6da15e96fb14e7428ecdc69306d8a3287296a968.tar.gz |
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>
-rw-r--r-- | 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 469cf53..654b705 100644 --- a/Documentation/devicetree/bindings/mmc/mmc.txt +++ b/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 |