|I2C Bus Arbitration
|While I2C supports multi-master buses this is difficult to get right.
|The implementation on the master side in software is quite complex.
|Clock-stretching and the arbitrary time that an I2C transaction can take
|make it difficult to share the bus fairly in the face of high traffic.
|When one or more masters can be reset independently part-way through a
|transaction it is hard to know the state of the bus.
|U-Boot provides a scheme based on two 'claim' GPIOs, one driven by the
|AP (Application Processor, meaning the main CPU) and one driven by the EC
|(Embedded Controller, a small CPU aimed at handling system tasks). With
|these they can communicate and reliably share the bus. This scheme has
|minimal overhead and involves very little code. The scheme can survive
|reboots by either side without difficulty.
|Since U-Boot runs on the AP, the terminology used is 'our' claim GPIO,
|meaning the AP's, and 'their' claim GPIO, meaning the EC's. This terminology
|is used by the device tree bindings in Linux also.
|The driver is implemented as an I2C mux, as it is in Linux. See
|i2c-arb-gpio-challenge for the implementation.
|GPIO lines are shared between the AP and EC to manage the bus. The AP and EC
|each have a 'bus claim' line, which is an output that the other can see.
|- AP_CLAIM: output from AP, signalling to the EC that the AP wants the bus
|- EC_CLAIM: output from EC, signalling to the AP that the EC wants 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 the AP wants to claim the bus. It:
|1. Asserts AP_CLAIM
|2. Waits a little bit for the other side to notice (slew time)
|3. Checks EC_CLAIM. If this is not asserted, then the AP has the bus, and we
| are done
|4. Otherwise, wait for a few milliseconds (retry time) and see if EC_CLAIM is
|5. If not, back off, release the claim and wait for a few more milliseconds
| (retry time again)
|6. Go back to 1 if things don't look wedged (wait time has expired)
|7. Panic. The other side is hung with the CLAIM line set.
|The same algorithm applies on the EC.
|To release the bus, just de-assert the claim line.
|Typical delays are:
|- slew time 10 us
|- retry time 3 ms
|- wait time - 50ms
|In general the traffic is fairly light, and in particular the EC wants access
|to the bus quite rarely (maybe every 10s or 30s to check the battery). This
|scheme works very nicely with very low contention. There is only a 10 us
|wait for access to the bus assuming that the other side isn't using it.