Fwd: umodem (4) recognize a CDC-ACM device

Bernd Walter ticso at cicely7.cicely.de
Thu Jun 3 10:10:35 UTC 2010


On Wed, Jun 02, 2010 at 06:25:25PM +0200, Hans Petter Selasky wrote:
> > > Hi,
> > >
> > > Can you dump the USB descriptors of your device?
> > 
> > Hi, right now I can do this at "FreeBSD 8.0-RELEASE-p2 #8 r206060M",
> > but If you prefer tonight I can dump from 'freebsd-current' box.
> > 
> > Thank you
> 
> Hi,
> 
> The problem is that LOW speed does not support BULK transfers according to the 
> USB specification. I guess we could switch that support on, though I'd rather 
> stick with the spec.

I think the original sense is a bit outdated.
It was when people had a single full speed controller on their systems
and low speed transfers could easily drop overall performance, so
it made sense to prohibid bulk transfers on low speed.
Today however people have multiple channels and high speed chains where
performance is only a matter up to the next transaction translator and
even if they don't use a high speed hub it only harms performance on
a single (of many) usb low/full channels, while bandwidth hungry devices
usually are connected to other controllers.

On the other hand - I never understood why people want to implement
low speed devices.
It has some many restrictions and isn't realy cheaper to build than
full speed devices.

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.


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