linux-kernel - Re: [PATCH 5/5] usb_debug: EXPERIMENTAL

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Message-ID: <Pine.LNX.4.44L0.0905071229270.3151-100000@iolanthe.rowland.org>
Date:	Thu, 7 May 2009 12:32:07 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jason Wessel <jason.wessel@...driver.com>, <greg@...ah.com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] usb_debug: EXPERIMENTAL - poll hcd device to force
 writes

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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