[linux-usb-devel] Dynamic USB buffer sizes (Was: Option driver enhancement)

Oliver Neukum oliver at neukum.org
Thu Oct 4 11:00:06 UTC 2007

Am Donnerstag 04 Oktober 2007 schrieb Matthias Urlichs:
> Hi,
> Indan Zupancic:
> > E.g. some mechanism where the buffer size and buffer count is chosen
> > dynamically depending on how fast the device turns out to be, and some
> > latency constraints.
> You can't measure how fast the device is. If you send too much too fast,
> you simply get dropped or mangled data at the other end of the data link;
> if you're doing PPP, completely wedging the channel, requiring a
> reconnect or even a powercycle of the card to recover, is not at all
> unlikely in my experience. :-(

OK, then you really don't have that option.

> I've even observed re-ordered data, which is why N_OUT_URB is currently
> set to 1 -- which (together with OUT_BUFLEN=128, 'cause the damn card
> sometimes dropped data with =256) *really* limits the send rate. Doesn't
> matter for plain-and-simple GSM, but beyond that ... ouch.

Hm. You should still get into every second frame ~500Hz, about
60 KB/s

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