3 ms·

What are the degrees of freedom in the spec that are resulting in millions of permutations of optional features?

rhn_mk1 8y ago
Not millions, but: - Power Delivery supports some 5 different maximum values - 3 different Battery Charging modes - 5 alternate modes according to Wikipedia (C only) - >10 different plugs (not an independent variable though) - 6 different signalling rates All of those are baked in in the hardware layer. It gets worse when you realize that cables have to support the right combination. It gets even worse when including things that piggyback on USB, like dual-purpose HDMI and USB ports, Thunderbolt, noncompliant chargers (Apple), Quick Charge, audio alternate mode.
WhitneyLand 8y ago
In addition to what RHN said, you've got to count absence of some of those features as another combination so you've got whatever power limits there are and also no power. You've also got cable lengths which I don't believe are limited by the spec but technologically limited. For example you can by 10 foot cables on Amazon but I believe it's currently impossible to buy any 10 foot cable there that has all the features supported that the spec allows for. Effectively this adds just as much complexity as something formally defined as a feature because end users have to deal with the additional variables just the same. I'm really trying to avoid doing enough research and combinatorics that would allow for an exact number but look, we're already at thousands which is ridiculous and you know how quickly the numbers grow with only a handful of additional headaches added in. We're also not counting edge cases like Apples Adapters that are supposed to allow an iOS device to charge while using USB accessories. That's another niche batch of secrets introduced with the 2018 iPad Pros. It only charges using certain combinations not simple requirements like having enough power.

Not millions, but: - Power Delivery supports some 5 different maximum values - 3 different Battery Charging modes - 5 alternate modes according to Wikipedia (C only) - >10 different plugs (not an independent variable though) - 6 different signalling rates All of those are baked in in the hardware layer. It gets worse when you realize that cables have to support the right combination. It gets even worse when including things that piggyback on USB, like dual-purpose HDMI and USB ports, Thunderbolt, noncompliant chargers (Apple), Quick Charge, audio alternate mode.

WhitneyLand 8y ago

In addition to what RHN said, you've got to count absence of some of those features as another combination so you've got whatever power limits there are and also no power. You've also got cable lengths which I don't believe are limited by the spec but technologically limited. For example you can by 10 foot cables on Amazon but I believe it's currently impossible to buy any 10 foot cable there that has all the features supported that the spec allows for. Effectively this adds just as much complexity as something formally defined as a feature because end users have to deal with the additional variables just the same. I'm really trying to avoid doing enough research and combinatorics that would allow for an exact number but look, we're already at thousands which is ridiculous and you know how quickly the numbers grow with only a handful of additional headaches added in. We're also not counting edge cases like Apples Adapters that are supposed to allow an iOS device to charge while using USB accessories. That's another niche batch of secrets introduced with the 2018 iPad Pros. It only charges using certain combinations not simple requirements like having enough power.