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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;
};
};
};
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