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| 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 <[email protected]> |
| 11 | + |
| 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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