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by littleJeck 1549 days ago
> Now I²C is a particularly sloppy protocol

I know this pain as an embedded software monkey. So many edge cases unthought of and no method to gracefully fail. Not to mention you always find some fault with the I²C implementation on the SOC or slave device you want to talk to.

I’ve spent weeks debugging bus lockup problems only to find erratas for the silicon, or find this particular chip needs an extra 100ms to startup than others.

It’s the one communication protocol that o always have problems with. Nothing else has caused me as many headaches.
1 comments
a9h74j 1549 days ago
This is something which worries me with Oxide using I2C (according to their board bring-up podcast), and using an auto-direction-switching level-shifter on the I2C bus. I hope and I am sure they can make it work, but to me that comes closer to violating a (core?) value of determinism and observability. Of course if it is only one well-studied component or two, perhaps out of design/supply necessity, this might be a very small risk in the larger scope.
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bcantrill 1548 days ago
It is indeed a very small risk in the larger scope -- and besides, using I2C isn't optional: all of the devices on the board (from converters/regulators, temp sensors, clock generators and fan controllers to CPUs, DIMMs, NICs, and NVMe drives) have I2C (or I2C-based) management interfaces. If you'd like more concrete detail, take a look at the Hubris app definition for our Gimlet board.[0]

[0] https://github.com/oxidecomputer/hubris/blob/master/app/giml...

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a9h74j 1547 days ago
Yes, clearly I2C, properly employed, adds far more to observability than it might, in principle, very rarely take away. And of course: a) Oxide is not the first to use the same components; b) Oxide is past the point of bring-up problems with a previously untried component; c) Oxide will have done all sorts of power cycling and confidence-building; so d) common-mode failures can likely be ruled out; and e) (I presume) any rare bus hang or error can itself be detected and reported.
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AdamH12113 1549 days ago
Are you sure it’s really I2C? Servers often use PMBus for power supply management, which uses the I2C physical layer but the (better-behaved) SMBus link layer.

I’m not familiar with Oxide, but the I2C physical layer is just a pair of open-drain signals, so I wouldn’t worry about a level-shifter too much.

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