[Nut-upsuser] lost USB comms under FreeBSD

Daniel O'Connor doconnor at gsoft.com.au
Thu Aug 23 01:43:22 UTC 2007

On Wed, 22 Aug 2007, Charles Lepple wrote:
> NUT drivers poll the UPS every so often, which works fine on systems
> where a single libusb call results in a single USB transaction. In
> FreeBSD (and probably other BSD systems as well), interrupt transfers
> are scheduled periodically, even if the calling program is not
> requesting interrupt transfers at the same rate.

I had a look through the USB code and it seems that ugen uses the 
default value which means that it obtains the value from the end point 

> What I saw with tripplite_usb (where the USB descriptor for the
> device tells the OS to poll 10-100 times a second) is that the driver
> does not poll frequently enough, and an interrupt transfer buffer
> fills up in the OS. Due to the way that libusb and the kernel USB
> stack interact, this could cause the symptoms you are seeing.

Hmm, which buffer?
It would be nice if it could recover gracefully from this situation 
though (since a number of things could cause missed interrupts anyway)

> I don't know of a good way to debug this. It seems like a problem in
> the kernel, but it's hard to isolate it with a driver as complex as
> usbhid-ups.

Yes :(
The USB stack is poorly documented too.

Daniel O'Connor software and network engineer
for Genesis Software - http://www.gsoft.com.au
"The nice thing about standards is that there
are so many of them to choose from."
  -- Andrew Tanenbaum
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