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Comment by PaulHoule | Hacker News Reader

USB3, USB-C is no answer.

Back when the USB1 spec came out it said you could plug 127 devices into a port through a hierarchy of hubs.

With three different laptops and a plethora of USB 3 hubs I found the system would support only a limited number of devices in the tree. If you plugged in too many devices (somewhere between 3 and 5), plugging in an new device would cause an existing device to drop out. It's just annoying if it is a keyboard or mouse that gets dropped out but if it is a storage device data could get corrupted.

I looked at the USB 3 spec and couldn't find any guarantee that there was any number of devices you could plug into hubs and have it work.

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> I looked at the USB 3 spec and couldn't find any guarantee that there was any number of devices you could plug into hubs and have it work.

It's a controller and/or BIOS limitation[1][2]. The complication is that while USB devices can be addressed using 7 bits = 127 devices total, each device usually creates more than one endpoint, and each endpoint consumes controller and system memory resources. The BIOS allocates the memory, and the amount is apparently hardcoded (guess a setting would be too difficult). If you have many USB 3 devices with a lot of endpoints, that memory runs out quickly.

In addition, each endpoint reserves some bandwidth, so the uplink needs to be able to provide that bandwidth.

[1]: https://community.intel.com/t5/Embedded-Intel-Core-Processor...

[2]: https://borncity.com/win/2019/09/06/windows-10-not-enough-us...

Now that you mention multiple endpoints, I noticed (admittedly, this is in USB1/2, which seems to be a totally separate system from USB3) my KB shows up as multiple HIDs. Probably for RGB and nkey rollover. But I wonder how it is implemented? Is there an internal hub (adding one more layer of hubs, ontop of whatever is done internally in my PC before the physical USB port - a lot, if lsusb/lspci-tree-list is to go by)?

USB Composite Device. for example https://www.tracesystemsinc.com/blog/composite-devices.html

Quite honestly USB 3 has always been flaky for me, I avoid using USB 3 hubs because they don't work reliably anyway, and on many cases the front USB 3 ports cause errors because... I don't even know. To say nothing of using longer cables than 50 cm. Back in the late 2000s USB 2.0 was similarly troublesome as USB 3 remains today. Slow USB 1.1 devices always seem to work though.

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