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+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/i2c/i2c-arb-gpio-challenge.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: GPIO-based I2C Arbitration Using a Challenge & Response Mechanism
+
+maintainers:
+ - Doug Anderson <dianders@chromium.org>
+ - Peter Rosin <peda@axentia.se>
+
+description: |
+ 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).
+
+properties:
+ compatible:
+ const: i2c-arb-gpio-challenge
+
+ i2c-parent:
+ $ref: /schemas/types.yaml#/definitions/phandle
+ description:
+ The I2C bus that this multiplexer's master-side port is connected to.
+
+ our-claim-gpios:
+ maxItems: 1
+ description:
+ The GPIO that we use to claim the bus.
+
+ slew-delay-us:
+ default: 10
+ description:
+ Time to wait for a GPIO to go high.
+
+ their-claim-gpios:
+ minItems: 1
+ maxItems: 8
+ description:
+ The GPIOs that the other sides use to claim the bus. Note that some
+ implementations may only support a single other master.
+
+ wait-free-us:
+ default: 50000
+ description:
+ We'll give up after this many microseconds.
+
+ wait-retry-us:
+ default: 3000
+ description:
+ We'll attempt another claim after this many microseconds.
+
+ i2c-arb:
+ type: object
+ $ref: /schemas/i2c/i2c-controller.yaml
+ unevaluatedProperties: false
+ description:
+ I2C arbitration bus node.
+
+required:
+ - compatible
+ - i2c-arb
+ - our-claim-gpios
+ - their-claim-gpios
+
+additionalProperties: false
+
+examples:
+ - |
+ #include <dt-bindings/gpio/gpio.h>
+ #include <dt-bindings/interrupt-controller/irq.h>
+
+ i2c-arbitrator {
+ compatible = "i2c-arb-gpio-challenge";
+ i2c-parent = <&i2c_4>;
+
+ our-claim-gpios = <&gpf0 3 GPIO_ACTIVE_LOW>;
+ their-claim-gpios = <&gpe0 4 GPIO_ACTIVE_LOW>;
+ slew-delay-us = <10>;
+ wait-retry-us = <3000>;
+ wait-free-us = <50000>;
+
+ i2c-arb {
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ sbs-battery@b {
+ compatible = "sbs,sbs-battery";
+ reg = <0xb>;
+ sbs,poll-retry-count = <1>;
+ };
+
+ embedded-controller@1e {
+ compatible = "google,cros-ec-i2c";
+ reg = <0x1e>;
+ interrupts = <6 IRQ_TYPE_LEVEL_HIGH>;
+ interrupt-parent = <&gpx1>;
+ pinctrl-names = "default";
+ pinctrl-0 = <&ec_irq>;
+ wakeup-source;
+ };
+ };
+ };