README.i2c 2.6 KB

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