4 ms·

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

by a9h74j 5y 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.

AdamH12113 5y 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.
bcantrill 5y 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/gimlet/rev-b.toml https://github.com/oxidecomputer/hubris/blob/master/app/giml...
- a9h74j 5y 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.

by a9h74j 5y ago

AdamH12113 5y 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.

bcantrill 5y 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/gimlet/rev-b.toml https://github.com/oxidecomputer/hubris/blob/master/app/giml...

a9h74j 5y 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.