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