Increase limit on ttyACMx devices on BeagleBone?

Hi all,

How do I get Debian to simultaneously connect to more than 6 ttyACMx USB serial devices simultaneously?

Setup:
BeagleBone Black
Pre-built Debian 10, 11, 12 images from the BeagleBone software images page (and forum)

All 8 devices show up properly as: /dev/ttyACM[0..7]

All three versions of Debian get the below error on opening the 7th device:
OSError: [Errno 5] Input/output error: '/dev/ttyACM6'

It doesn't matter which specific ttyACMx devices are opened, but as soon as the 6th one is opened, no additional can be opened.

I've tried specifying an increased number of 8250.nr_uarts and nr_uarts via kernel parameters, and that helped w/ getting additional 8250 UARTS, but that didn't allow any more simultaneous ttyACMx connections.
These ttyACM devices are using the kernel driver cdc_acm. I can't find any info on any limitations with this driver.

As an aside: We tried connecting the same USB hub and devices up to a Windows 11 and Red Hat Enterprise Linux 9 laptop, and on those systems, we can have simultaneous connections to all 8 devices.

Thanks in advance.

Statistics: Posted by updog — 2025-02-06 23:25 — Replies 1 — Views 35

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