Re: [PATCH 5/5] usb_debug: EXPERIMENTAL - poll hcd device to force writes

[Oliver Neukum <oliver@neukum.org>]

Am Donnerstag, 7. Mai 2009 16:35:57 schrieb Alan Stern:
> On Thu, 7 May 2009, Oliver Neukum wrote:
> > On a fundamental note, thinking about this in terms of numbers of URBs
> > is strictly speaking wrong. We need to limit data in flight. For
> > efficiency we should make buffers as large as possible within that limit.
>
> But for latency you should submit URBs as soone as possible within that
> limit, which generally means small buffers.

This is true only if the device's buffer would run dry. But if no URB is
in flight, a URB should be written right away.

> How about setting the upper limit to URBs in flight based on the baud
> rate?  Faster transfers deserve more URBs, right?  Assuming some
> minimum number of bytes per URB (4? 8?), there should be enough URBs to
> fill a pipeline whose length is around 5 ms or so (interrupt latency).

Hm, you say many URBs can complete before an interrupt handler
can react?

	Regards
		Oliver