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

[Hans Petter Selasky]

On Tuesday 15 November 2011 22:21:31 mdf at freebsd.org wrote:
> On Tue, Nov 15, 2011 at 1:20 PM,  <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.
> 
> ... and I also just remembered that I have seen recommendations that,
> when FreeBSD is used as a virtual machine, hz should be set to 100 so
> that the virtual interrupt overhead is reduced.  Those two
> recommendations are at odds with each other.

USB in virtual machines is a completely different story. All the timing 
critical stuff is handled by the real USB host operating system. The OS inside 
the VM just has to respond to some "virtual" USB events. Timing does not 
matter very much as far as I'm aware.

--HPS