On Sat, Jul 22, 2023 at 11:57:10AM +0200, Krzysztof Kozlowski wrote: > +description: | > + This uses GPIO lines and a challenge & response mechanism to arbitrate who is > + the master of an I2C bus in a multimaster situation. > + > + In many cases using GPIOs to arbitrate is not needed and a design can use the > + standard I2C multi-master rules. Using GPIOs is generally useful in the case > + where there is a device on the bus that has errata and/or bugs that makes > + standard multimaster mode not feasible. > + > + Note that this scheme works well enough but has some downsides: > + * It is nonstandard (not using standard I2C multimaster) > + * Having two masters on a bus in general makes it relatively hard to debug > + problems (hard to tell if i2c issues were caused by one master, another, > + or some device on the bus). > + > + Algorithm: > + All masters on the bus have a 'bus claim' line which is an output that the > + others can see. These are all active low with pull-ups enabled. We'll > + describe these lines as: > + * OUR_CLAIM: output from us signaling to other hosts that we want the bus > + * THEIR_CLAIMS: output from others signaling that they want 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 we want to claim the bus. We: > + 1. Assert OUR_CLAIM. > + 2. Waits a little bit for the other sides to notice (slew time, say 10 > + microseconds). > + 3. Check THEIR_CLAIMS. If none are asserted then the we have the bus and we > + are done. > + 4. Otherwise, wait for a few milliseconds and see if THEIR_CLAIMS are released. > + 5. If not, back off, release the claim and wait for a few more milliseconds. > + 6. Go back to 1 (until retry time has expired). > + their-claim-gpios: > + minItems: 1 > + maxItems: 2 > + description: > + The GPIOs that the other sides use to claim the bus. Note that some > + implementations may only support a single other master. Where does the maxItems: 2 come from?
Attachment:
signature.asc
Description: PGP signature
