upstream u-boot with additional patches for our devices/boards:
https://lists.denx.de/pipermail/u-boot/2017-March/282789.html (AXP crashes) ;
Gbit ethernet patch for some LIME2 revisions ;
with SPI flash support
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
61 lines
2.6 KiB
61 lines
2.6 KiB
9 years ago
|
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
|
||
|
|
released
|
||
|
|
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.
|