USB Serial port woes [LWN.net]

USB Serial port woes

Posted Jun 26, 2011 16:31 UTC (Sun) by speedster1 (guest, #8143)
In reply to: User-friendly disk names by mikov
Parent article: User-friendly disk names

> A single multi-port USB to serial converter would address this nicely,
> but we have evaluated about a dozen of them, and while all they work
> perfectly in Windows, they are either unsupported in Linux at all,
> or horribly broken (like dropping a byte every couple of hours).
> It is an embarrassing shame. I wish our company had the resources
> to sponsor somebody to fix that.

Do they _really_ work perfectly in Windows? In my experience, cheap serial converters are prone to dropping a byte now and then, and when I searched online it turned out that it was hardware not drivers -- Windows users would experience the same problem given the same level of usage and attention to detail (it just happens a lot less often, because there are more of us Linux developers making heavy use of serial ports).

Sorry I can't find an actual reference explaining the hardware issue on a common PLX usb-serial chip; there is no sufficient replacement yet for the recently-discontinued Google Linux search :`-(

I agree that keyspan did not have these problems, but they were significantly more expensive and seem to have lost out to the even cheaper but somewhat flaky adapters...

If there are current multi-port adapters that really do have higher quality chips in them (i.e. like keyspan rather than PLX) but lack Linux drivers, then somebody should get in touch with the Linux Driver Project and donate such an adapter!
http://www.linuxdriverproject.org/

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USB Serial port woes

Posted Jun 27, 2011 3:34 UTC (Mon) by mikov (guest, #33179) [Link] (2 responses)

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.

USB Serial port woes

Posted Jun 27, 2011 16:35 UTC (Mon) by speedster1 (guest, #8143) [Link] (1 responses)

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 (guest, #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.