projects / deliverable / linux.git / blame
| Commit | Line | Data |
|---|---|---|
| b81dfaa0 DA |
1 | GPIO-based I2C Arbitration Using a Challenge & Response Mechanism |
| 2 | ================================================================= | |
| 3 | This uses GPIO lines and a challenge & response mechanism to arbitrate who is | |
| 4 | the master of an I2C bus in a multimaster situation. | |
| 5 | ||
| 6 | In many cases using GPIOs to arbitrate is not needed and a design can use | |
| 7 | the standard I2C multi-master rules. Using GPIOs is generally useful in | |
| 8 | the case where there is a device on the bus that has errata and/or bugs | |
| 9 | that makes standard multimaster mode not feasible. | |
| 10 | ||
| a83bea7c DA |
11 | Note that this scheme works well enough but has some downsides: |
| 12 | * It is nonstandard (not using standard I2C multimaster) | |
| 13 | * Having two masters on a bus in general makes it relatively hard to debug | |
| 14 | problems (hard to tell if i2c issues were caused by one master, another, or | |
| 15 | some device on the bus). | |
| 16 | ||
| b81dfaa0 DA |
17 | |
| 18 | Algorithm: | |
| 19 | ||
| 20 | All masters on the bus have a 'bus claim' line which is an output that the | |
| 21 | others can see. These are all active low with pull-ups enabled. We'll | |
| 22 | describe these lines as: | |
| 23 | ||
| 24 | - OUR_CLAIM: output from us signaling to other hosts that we want the bus | |
| 25 | - THEIR_CLAIMS: output from others signaling that they want the bus | |
| 26 | ||
| 27 | The basic algorithm is to assert your line when you want the bus, then make | |
| 28 | sure that the other side doesn't want it also. A detailed explanation is best | |
| 29 | done with an example. | |
| 30 | ||
| 31 | Let's say we want to claim the bus. We: | |
| 32 | 1. Assert OUR_CLAIM. | |
| 33 | 2. Waits a little bit for the other sides to notice (slew time, say 10 | |
| 34 | microseconds). | |
| 35 | 3. Check THEIR_CLAIMS. If none are asserted then the we have the bus and we are | |
| 36 | done. | |
| 37 | 4. Otherwise, wait for a few milliseconds and see if THEIR_CLAIMS are released. | |
| 38 | 5. If not, back off, release the claim and wait for a few more milliseconds. | |
| 39 | 6. Go back to 1 (until retry time has expired). | |
| 40 | ||
| 41 | ||
| 42 | Required properties: | |
| 43 | - compatible: i2c-arb-gpio-challenge | |
| 44 | - our-claim-gpio: The GPIO that we use to claim the bus. | |
| 45 | - their-claim-gpios: The GPIOs that the other sides use to claim the bus. | |
| 46 | Note that some implementations may only support a single other master. | |
| e8813c15 | 47 | - I2C arbitration bus node. See i2c-arb.txt in this directory. |
| b81dfaa0 DA |
48 | |
| 49 | Optional properties: | |
| 50 | - slew-delay-us: microseconds to wait for a GPIO to go high. Default is 10 us. | |
| 51 | - wait-retry-us: we'll attempt another claim after this many microseconds. | |
| 52 | Default is 3000 us. | |
| 53 | - wait-free-us: we'll give up after this many microseconds. Default is 50000 us. | |
| 54 | ||
| 55 | ||
| 56 | Example: | |
| 57 | i2c@12CA0000 { | |
| 58 | compatible = "acme,some-i2c-device"; | |
| 59 | #address-cells = <1>; | |
| 60 | #size-cells = <0>; | |
| 61 | }; | |
| 62 | ||
| 63 | i2c-arbitrator { | |
| 64 | compatible = "i2c-arb-gpio-challenge"; | |
| b81dfaa0 DA |
65 | |
| 66 | i2c-parent = <&{/i2c@12CA0000}>; | |
| 67 | ||
| 68 | our-claim-gpio = <&gpf0 3 1>; | |
| 69 | their-claim-gpios = <&gpe0 4 1>; | |
| 70 | slew-delay-us = <10>; | |
| 71 | wait-retry-us = <3000>; | |
| 72 | wait-free-us = <50000>; | |
| 73 | ||
| e8813c15 | 74 | i2c-arb { |
| b81dfaa0 DA |
75 | #address-cells = <1>; |
| 76 | #size-cells = <0>; | |
| 77 | ||
| 78 | i2c@52 { | |
| 79 | // Normal I2C device | |
| 80 | }; | |
| 81 | }; | |
| 82 | }; |