Re: Number of USB devices limited?

Not specific to FreeBSD, but to the USB standard itself (I think). There 
is a limit to the number of endpoints, as you describe, but there is 
also a limit to the number of hubs you can cascade. The number is low; I 
think it's 7. If you have more than $NUMBER of hubs in a row, you will 
get erratic behavior from your USB endpoints. And many things count as a 
"hub" - your computer probably takes up two, for example.


On Tue, 19 Apr 2022, Brian McGovern (bmcgover) wrote:

> Looking at the FreeBSD handbook, Chapter 13, it looks like the 
> expectation is that I should be able to attach "the maximum of 126 
> devices". Unfortunately, it seems I'm getting in the low 20s (21 FTDI 
> serial cables, USB keyboard, and mouse), after which I start getting 
> the following in the logs:
>
> "Error initializing deviceusb_alloc_device: device init 34 failed 
> (USB_ERR_IOERROR, ignored)"
>
> and
>
> "uhub_reattach_port: could not allocate new device".
>
>
> If I look at /dev/ugen*, there are the ugen devices for 0.1->0.33; so 
> its feeling like there may be limit in the serial port driver for the 
> number of /dev/cua* devices, which presently only number 
> cuaU0->cuaU20. So, I'm assuming this may not be a "USB" issue as much 
> as the number of USB serial ports (ucom?) that can be connected.
>
> Before I go on a code crawl to figure out how this is failing and if 
> it can be scaled up, I'm curious if anyone is aware of a coded limit 
> for the number of USB serial devices (or USB devices in general) that 
> can be connected before running in to this? I'm looking to push 
> upwards to _maybe_ 40 FTDI devices.
>
>          -Brian
>

-- 
Chris Hill               chris@monochrome.org