[LAU] USB on ThinkPad T520

Monty Montgomery xiphmont at gmail.com
Sun May 13 19:28:37 UTC 2012

> The idea is to use the uhci_hcd (USB1) driver instead of ehci_hcd
> (USB2). -- Maybe the module is not loaded?

You do not have a UHCI controller.  You only have EHCI controllers.
On your machine, all USB 1.1 is handled by transaction translators in
your EHCI controller's root hub.

The UHCI driver is not a USB 1.1 driver-- it's a driver for UHCI
hardware which happens to be for USB 1.1, and which you do not have at
all.  Older machines paired a UHCI and EHCI controller to handle USB
1.1 and 2.0; newer machines use EHCI for both.

The linux EHCI TT scheduler is not very good.  I wrote a new one from
scratch a few years ago, but the kernel people considered it low
priority and I got sick of maintaining it against updates, as they
dropped it from testing kernels whenever any other incoming patch
conflicted. OTOH, the EHCI hardware itself is much more reliable than
the UHCI hardware, which apparently suffers from isochronous dropouts
even when the driver functions perfectly.


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