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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
commit57f0f512b273f60d52568b8c6b77e17f5636edc0 (patch)
tree5e910f0e82173f4ef4f51111366a3f1299037a7b /Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt
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+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.
+
+Note that this scheme works well enough but has some downsides:
+* It is nonstandard (not using standard I2C multimaster)
+* Having two masters on a bus in general makes it relatively hard to debug
+ problems (hard to tell if i2c issues were caused by one master, another, or
+ some device on the bus).
+
+
+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
+ };
+ };
+ };