USB Serial port woes [LWN.net]

USB Serial port woes

Posted Jun 27, 2011 3:34 UTC (Mon) by mikov (subscriber, #33179)
In reply to: USB Serial port woes by speedster1
Parent article: User-friendly disk names

It is difficult for me to say how good they really were in Windows, since it didn't matter. We needed them in Linux and we only did tests in Windows to make sure a specific unit wasn't defective. They definitely did behave measurably better in Windows - after comparable amount of testing there weren't any dropped bytes.

There is one additional piece of information however, which can explain at least part of what I saw:

Most of the multiport USB-to-serial adapters are in fact a USB hub with ordinary single-port adapters hanging from it. The problem is that the hub is USB 2.0 while the serial adapters themselves are USB 1.0. Linux kernels at least up to 2.6.26 had a bug where USB 1.0 devices did not work reliably behind a USB 2.0 bridge.

I have complained about this problem before on LWN and I got advice to try other config settings, which I did, but saw no improvement.

I am not 100% sure the problems were caused by this and it is also possible that it has been fixed in later kernel versions.

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USB Serial port woes

Posted Jun 27, 2011 16:35 UTC (Mon) by speedster1 (subscriber, #8143) [Link]

Here is a complaint about single-port Prolific PL2303 adapters, with which I've experienced reliability issues (note I meant to say "Prolific" not "PLX" in previous comment; PLX makes perfectly good PCI bridges, not cheap USB chips). It makes the observation about dropped bytes, but doesn't have an explanation about why they are prone to occasional data loss; I think that was in a forum post not a mailing list post, and I still can't find it.

http://article.gmane.org/gmane.comp.monitoring.nut.user/5...

If your problem is instead a more generic 2.0 hub driver problem, could you post a link discussing it? With details, I might be able to go check commit messages to see if it has supposedly been fixed.

USB Serial port woes

Posted Jun 28, 2011 4:29 UTC (Tue) by mikov (subscriber, #33179) [Link]

OK, I will look for the link and post back here.

BTW, we have been using PL2303 USB-to-serial adapters and we haven't had any issues with them, as long as (important!) USB 2.0 is disabled.