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

From: Alan Stern
Date: Thu May 07 2009 - 12:32:22 EST


On Thu, 7 May 2009, Oliver Neukum wrote:

> > 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?

How long can interrupts remain disabled? On a non-RT system, it might
be several milliseconds.

In any case, a full-speed host controller won't issue IRQs more often
than once per ms. Quite a few URBs can complete in that time.

Alan Stern

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