1 # SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
2 %YAML 1.2
3 ---
4 $id: http://devicetree.org/schemas/i2c/i2c-arb-gpio-challenge.yaml#
5 $schema: http://devicetree.org/meta-schemas/core.yaml#
6
7 title: GPIO-based I2C Arbitration Using a Challenge & Response Mechanism
8
9 maintainers:
10 - Doug Anderson <dianders@chromium.org>
11 - Peter Rosin <peda@axentia.se>
12
13 description: |
14 This uses GPIO lines and a challenge & response mechanism to arbitrate who is
15 the master of an I2C bus in a multimaster situation.
16
17 In many cases using GPIOs to arbitrate is not needed and a design can use the
18 standard I2C multi-master rules. Using GPIOs is generally useful in the case
19 where there is a device on the bus that has errata and/or bugs that makes
20 standard multimaster mode not feasible.
21
22 Note that this scheme works well enough but has some downsides:
23 * It is nonstandard (not using standard I2C multimaster)
24 * Having two masters on a bus in general makes it relatively hard to debug
25 problems (hard to tell if i2c issues were caused by one master, another,
26 or some device on the bus).
27
28 Algorithm:
29 All masters on the bus have a 'bus claim' line which is an output that the
30 others can see. These are all active low with pull-ups enabled. We'll
31 describe these lines as:
32 * OUR_CLAIM: output from us signaling to other hosts that we want the bus
33 * THEIR_CLAIMS: output from others signaling that they want the bus
34
35 The basic algorithm is to assert your line when you want the bus, then make
36 sure that the other side doesn't want it also. A detailed explanation is
37 best done with an example.
38
39 Let's say we want to claim the bus. We:
40 1. Assert OUR_CLAIM.
41 2. Waits a little bit for the other sides to notice (slew time, say 10
42 microseconds).
43 3. Check THEIR_CLAIMS. If none are asserted then the we have the bus and we
44 are done.
45 4. Otherwise, wait for a few milliseconds and see if THEIR_CLAIMS are released.
46 5. If not, back off, release the claim and wait for a few more milliseconds.
47 6. Go back to 1 (until retry time has expired).
48
49 properties:
50 compatible:
51 const: i2c-arb-gpio-challenge
52
53 i2c-parent:
54 $ref: /schemas/types.yaml#/definitions/phandle
55 description:
56 The I2C bus that this multiplexer's master-side port is connected to.
57
58 our-claim-gpios:
59 maxItems: 1
60 description:
61 The GPIO that we use to claim the bus.
62
63 slew-delay-us:
64 default: 10
65 description:
66 Time to wait for a GPIO to go high.
67
68 their-claim-gpios:
69 minItems: 1
70 maxItems: 8
71 description:
72 The GPIOs that the other sides use to claim the bus. Note that some
73 implementations may only support a single other master.
74
75 wait-free-us:
76 default: 50000
77 description:
78 We'll give up after this many microseconds.
79
80 wait-retry-us:
81 default: 3000
82 description:
83 We'll attempt another claim after this many microseconds.
84
85 i2c-arb:
86 type: object
87 $ref: /schemas/i2c/i2c-controller.yaml
88 unevaluatedProperties: false
89 description:
90 I2C arbitration bus node.
91
92 required:
93 - compatible
94 - i2c-arb
95 - our-claim-gpios
96 - their-claim-gpios
97
98 additionalProperties: false
99
100 examples:
101 - |
102 #include <dt-bindings/gpio/gpio.h>
103 #include <dt-bindings/interrupt-controller/irq.h>
104
105 i2c-arbitrator {
106 compatible = "i2c-arb-gpio-challenge";
107 i2c-parent = <&i2c_4>;
108
109 our-claim-gpios = <&gpf0 3 GPIO_ACTIVE_LOW>;
110 their-claim-gpios = <&gpe0 4 GPIO_ACTIVE_LOW>;
111 slew-delay-us = <10>;
112 wait-retry-us = <3000>;
113 wait-free-us = <50000>;
114
115 i2c-arb {
116 #address-cells = <1>;
117 #size-cells = <0>;
118
119 sbs-battery@b {
120 compatible = "sbs,sbs-battery";
121 reg = <0xb>;
122 sbs,poll-retry-count = <1>;
123 };
124
125 embedded-controller@1e {
126 compatible = "google,cros-ec-i2c";
127 reg = <0x1e>;
128 interrupts = <6 IRQ_TYPE_LEVEL_HIGH>;
129 interrupt-parent = <&gpx1>;
130 pinctrl-names = "default";
131 pinctrl-0 = <&ec_irq>;
132 wakeup-source;
133 };
134 };
135 };
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