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

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Message-Id: <200905071701.51195.oliver@neukum.org>
Date:	Thu, 7 May 2009 17:01:50 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Alan Stern <stern@...land.harvard.edu>
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

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

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