this post was submitted on 25 Mar 2024
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My work machine isn't too unusual, apart that it has 52 USB devices connected. And here's something you may not know: Linux can't enumerate more than 16 USB ports if the root is configured as USB3, so I had to force all the ports to run in USB2 mode - which is fine in this case, since most of them are serial ports.
What? Really?
I'm not sure it's a kernel limitation or a hardware limitation. But it does throw an error in syslog when you connect the 17th device. Not as USB2 though.
This is caused by your root controller's limited bandwidth and it's inability to handle that many 3.0 devices at the same time. Some of the newer motherboards with USB C PD have controllers in them that can do a lot more.
It's basically a hack on part of the company that made the root controller IC. They know they only have enough internal bandwidth to support 16 USB 3.0 devices so they intentionally bork things when you plug in more than that since their Transaction Translator (TT) can't handle more and they were too lazy to bother implementing the ability to share 2.0 and 3.0 properly.
I'm guessing the decision went something like this...
"We have enough bandwidth for 16 3.0 devices... What do we do if someone plugs in more than that?" "Only a few people will ever have that many! We don't have the budget to handle every tiny little use case! Just ship it."
So it's not Linux fault in this case. Or at least, if it is (a problem with the driver) it's because of some proprietary bullshit that the driver requires to function properly 🤷
Yeah I figured it might be something like that. But I wasn't sure it wasn't a kernel limit - or even a limit in the USB3 specification - because I actually only have one USB3-capable device connected (my cellphone). All the other devices are low-bandwidth USB2 FTDI USB serial converters. I thought it couldn't be a bandwidth issue when all but one device can only use a fraction of what's available.
Uh, you're outputing 52 DMX universes straight from USB? I have questions!
I had no idea what you're talking about so I had to look it up 🙂
I think you're making assumptions on what I need many serial ports for and it's nothing like what you think.
I work for a company that makes measuring instruments that talk serial (RS232, RS422 and UART), we have many variants of our products and I'm tired of plugging and unplugging devices to the same serial ports to test code. Also, I can't do that remotely when I work from home. So I have many serial ports and all the different devices I need to test my code on regularly are all plugged in and powered at the same time.
No lighting here 🙂
Oh, here I thought you was running a fancy dancefloor or something! TIL the creator of that chip was a prick.