| Tom Rini | 53633a8 | 2024-02-29 12:33:36 -0500 | [diff] [blame] | 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 | }; |