From b81dfaa01f7057dde4a6356740c023db4e2ce83b Mon Sep 17 00:00:00 2001 From: Doug Anderson Date: Tue, 16 Apr 2013 06:29:00 +0000 Subject: i2c: mux: Add i2c-arb-gpio-challenge 'mux' driver The i2c-arb-gpio-challenge driver implements an I2C arbitration scheme where masters need to claim the bus with a GPIO before they can start a transaction. This should generally only be used when standard I2C multimaster isn't appropriate for some reason (errata/bugs). This driver is based on code that Simon Glass added to the i2c-s3c2410 driver in the Chrome OS kernel 3.4 tree. The current incarnation as a mux driver is as suggested by Grant Likely. See for some history. Signed-off-by: Doug Anderson Signed-off-by: Simon Glass Signed-off-by: Naveen Krishna Chatradhi Reviewed-by: Stephen Warren Acked-by: Olof Johansson Signed-off-by: Wolfram Sang --- .../bindings/i2c/i2c-arb-gpio-challenge.txt | 80 ++++++++++++++++++++++ 1 file changed, 80 insertions(+) create mode 100644 Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt (limited to 'Documentation') diff --git a/Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt b/Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt new file mode 100644 index 00000000000..1ac8ea8ade1 --- /dev/null +++ b/Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt @@ -0,0 +1,80 @@ +GPIO-based I2C Arbitration Using a Challenge & Response Mechanism +================================================================= +This uses GPIO lines and a challenge & response mechanism to arbitrate who is +the master of an I2C bus in a multimaster situation. + +In many cases using GPIOs to arbitrate is not needed and a design can use +the standard I2C multi-master rules. Using GPIOs is generally useful in +the case where there is a device on the bus that has errata and/or bugs +that makes standard multimaster mode not feasible. + + +Algorithm: + +All masters on the bus have a 'bus claim' line which is an output that the +others can see. These are all active low with pull-ups enabled. We'll +describe these lines as: + +- OUR_CLAIM: output from us signaling to other hosts that we want the bus +- THEIR_CLAIMS: output from others signaling that they want the bus + +The basic algorithm is to assert your line when you want the bus, then make +sure that the other side doesn't want it also. A detailed explanation is best +done with an example. + +Let's say we want to claim the bus. We: +1. Assert OUR_CLAIM. +2. Waits a little bit for the other sides to notice (slew time, say 10 + microseconds). +3. Check THEIR_CLAIMS. If none are asserted then the we have the bus and we are + done. +4. Otherwise, wait for a few milliseconds and see if THEIR_CLAIMS are released. +5. If not, back off, release the claim and wait for a few more milliseconds. +6. Go back to 1 (until retry time has expired). + + +Required properties: +- compatible: i2c-arb-gpio-challenge +- our-claim-gpio: The GPIO that we use to claim the bus. +- their-claim-gpios: The GPIOs that the other sides use to claim the bus. + Note that some implementations may only support a single other master. +- Standard I2C mux properties. See mux.txt in this directory. +- Single I2C child bus node at reg 0. See mux.txt in this directory. + +Optional properties: +- slew-delay-us: microseconds to wait for a GPIO to go high. Default is 10 us. +- wait-retry-us: we'll attempt another claim after this many microseconds. + Default is 3000 us. +- wait-free-us: we'll give up after this many microseconds. Default is 50000 us. + + +Example: + i2c@12CA0000 { + compatible = "acme,some-i2c-device"; + #address-cells = <1>; + #size-cells = <0>; + }; + + i2c-arbitrator { + compatible = "i2c-arb-gpio-challenge"; + #address-cells = <1>; + #size-cells = <0>; + + i2c-parent = <&{/i2c@12CA0000}>; + + our-claim-gpio = <&gpf0 3 1>; + their-claim-gpios = <&gpe0 4 1>; + slew-delay-us = <10>; + wait-retry-us = <3000>; + wait-free-us = <50000>; + + i2c@0 { + reg = <0>; + #address-cells = <1>; + #size-cells = <0>; + + i2c@52 { + // Normal I2C device + }; + }; + }; -- cgit v1.2.3-18-g5258