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12 comments
basisbit 🦈🇪🇺🇺🇦

@da_667 @catsalad
The standard also says a maximum depth of usb hubs of 3 afafik

Biggles

@basisbit @da_667 @catsalad even then it is unreliable e.g. laptops like to build hubs into themselves (we have this problem with the work setups)

basisbit 🦈🇪🇺🇺🇦

@Taco_lad @da_667 @catsalad
Usually I would not link to him, but.... LinusTechTips recently made a video explaining this a bit:
youtube.com/watch?v=hiwaxlttWo #usb

Agnieszka R. Turczyńska

@Taco_lad @basisbit @da_667 @catsalad I have shared the picture with a friend, and he answered: "You want to kill me". Then he shared the picture with his friends and get back to me with some informations. I will use that as an excuse to enlighten you. However, if you are already USB expert, you may safely skip to the end of this toot.

So, this is how it goes.
This is maximum you can achieve using those hubs. You can have 7 layers of devices in USB, however host and root count as a layer themselves. That lefts us 5 layers to use. Sometimes the device has a hub build inside, which takes another layer.

Each hub can have 7 ports at most, but because of reasons (unknown to me) hubs with more than 4 ports have two 4-ports chained together.

In theory, using the device tree as effectively as possible, one can connect 117,649 devices to the single controller, however there is a limit implemented in many of the controllers, as they can handle up to 128 devices.

Allegedly, the newest AMD chipsets can handle 512 devices.

So, this is what I've learned today. And if I need this knowledge even after, that means something went really, really wrong.

Enjoy your day, fellow nerdy people.

PS. Does anyone know a technique to remember the number 117,649?

@Taco_lad @basisbit @da_667 @catsalad I have shared the picture with a friend, and he answered: "You want to kill me". Then he shared the picture with his friends and get back to me with some informations. I will use that as an excuse to enlighten you. However, if you are already USB expert, you may safely skip to the end of this toot.

HansvR

@agturcz I would just remember it, but tomorrow I've forgotten your name.

David JONES

@basisbit @da_667 @catsalad I think you could achieve that with an even more cursed arrangement.

artemist

@basisbit @da_667 @catsalad The max depth is 7 and it's somewhat artificial, if you patch the kernel you can get around it.

✧✦✶✷Catherine✷✶✦✧

@artemist @basisbit @da_667 @catsalad for USB2 it's based on turnaround time and cable length; it's not really artificial so much as there's margin available (which you can hack away)

[SUYA] ひみつ :blabcatverified:
@da_667 @catsalad the standard also says max 7 devices in a chain (pc + 5 hubs + 1 end device in this case)

although I think they're called classes or something in the spec, I don't really remember
Noodles

@da_667 @catsalad For some reason, I have a very distinct memory of @leo talking about this on the Screen Savers in like, the 90s, and I am still, to this day, disappointed that every USB device does not include a port allowing you to just, continuously daisy chain Mice and Keyboards.

Instead, modern computers can barely be assed to give us one port on the machine itself.

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