svn commit: r227541 - head/sys/dev/usb/controller

[Hans Petter Selasky]

On Tuesday 15 November 2011 22:20:18 mdf at freebsd.org wrote:
> On Tue, Nov 15, 2011 at 1:02 PM, Hans Petter Selasky <hselasky at c2i.net> 
wrote:
> > For USB compliant operation, the USB stack requires hz to be greater or
> > equal to 250 hz, to put it like that. Mostly a requirement in USB
> > gadget/device mode.
> 
> Really?  That's news to me.  Is that documented somewhere?  I know we
> still use hz=100 internally, but we're on stable/7 still so not using
> the new USB stack yet.

No it is not documented anywhere. This delay is mostly critical if you enable 
USB power saving features like suspend and resume. Then there are some 
software timers which should not derive too much.

Most of the time the delays in USB are not critical. Transfer timers are in 
the seconds range and that works fine with hz=100.

Where and how should I document such are requirement?

Add something during system init?

if (hz < 250)
   printf("USB: hz is too low (ignored)\n");

--HPS