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      1 I2C Bus Arbitration
      2 ===================
      3 
      4 While I2C supports multi-master buses this is difficult to get right.
      5 The implementation on the master side in software is quite complex.
      6 Clock-stretching and the arbitrary time that an I2C transaction can take
      7 make it difficult to share the bus fairly in the face of high traffic.
      8 When one or more masters can be reset independently part-way through a
      9 transaction it is hard to know the state of the bus.
     10 
     11 U-Boot provides a scheme based on two 'claim' GPIOs, one driven by the
     12 AP (Application Processor, meaning the main CPU) and one driven by the EC
     13 (Embedded Controller, a small CPU aimed at handling system tasks). With
     14 these they can communicate and reliably share the bus. This scheme has
     15 minimal overhead and involves very little code. The scheme can survive
     16 reboots by either side without difficulty.
     17 
     18 Since U-Boot runs on the AP, the terminology used is 'our' claim GPIO,
     19 meaning the AP's, and 'their' claim GPIO, meaning the EC's. This terminology
     20 is used by the device tree bindings in Linux also.
     21 
     22 The driver is implemented as an I2C mux, as it is in Linux. See
     23 i2c-arb-gpio-challenge for the implementation.
     24 
     25 GPIO lines are shared between the AP and EC to manage the bus. The AP and EC
     26 each have a 'bus claim' line, which is an output that the other can see.
     27 
     28 - AP_CLAIM: output from AP, signalling to the EC that the AP wants the bus
     29 - EC_CLAIM: output from EC, signalling to the AP that the EC wants the bus
     30 
     31 The basic algorithm is to assert your line when you want the bus, then make
     32 sure that the other side doesn't want it also. A detailed explanation is best
     33 done with an example.
     34 
     35 Let's say the AP wants to claim the bus. It:
     36 
     37 1. Asserts AP_CLAIM
     38 2. Waits a little bit for the other side to notice (slew time)
     39 3. Checks EC_CLAIM. If this is not asserted, then the AP has the bus, and we
     40    are done
     41 4. Otherwise, wait for a few milliseconds (retry time) and see if EC_CLAIM is
     42    released
     43 5. If not, back off, release the claim and wait for a few more milliseconds
     44   (retry time again)
     45 6. Go back to 1 if things don't look wedged (wait time has expired)
     46 7. Panic. The other side is hung with the CLAIM line set.
     47 
     48 The same algorithm applies on the EC.
     49 
     50 To release the bus, just de-assert the claim line.
     51 
     52 Typical delays are:
     53 - slew time 10 us
     54 - retry time 3 ms
     55 - wait time - 50ms
     56 
     57 In general the traffic is fairly light, and in particular the EC wants access
     58 to the bus quite rarely (maybe every 10s or 30s to check the battery). This
     59 scheme works very nicely with very low contention. There is only a 10 us
     60 wait for access to the bus assuming that the other side isn't using it.
     61