linux-kernel - Re: [Patch] Increase USBFS Bulk Transfer size

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Date:	Mon, 7 Nov 2011 14:12:16 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
cc:	Tim Vlaar <Tvlaar@...rey.com>, Greg KH <greg@...ah.com>,
	Markus Rechberger <mrechberger@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	USB list <linux-usb@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] Increase USBFS Bulk Transfer size

On Mon, 7 Nov 2011, Sarah Sharp wrote:

> On Fri, Oct 14, 2011 at 08:33:29AM -0600, Greg KH wrote:
> > On Fri, Oct 14, 2011 at 10:05:41AM -0400, Alan Stern wrote:
> > > No, a much better approach is to remove all limits on individual
> > > transfer sizes and instead have a global limit on the total amount of
> > > all usbfs buffers in use at any time.  Maybe something like 16 MB; at 
> > > SuperSpeed, that's about about 30 ms worth of data.
> > 
> > That sounds quite reasonable.
> 
> Alan, won't this global limit on the usbfs URB buffer size effect
> userspace drivers that are currently allocating large amounts of
> buffers, but still respecting individual buffer limit of 16KB?  It seems
> like the patch has the potential to break userspace drivers.

It might indeed.  A further enhancement would replace that 16-MB global
constant with a sysfs attribute (a writable module parameter for
usbcore).  Do you have any better suggestions?

> I think that Point Grey's USB 3.0 webcam will be attempting to queue a
> series of bulk URBs that will be bigger than your 16MB global limit.

For SuperSpeed, 16 MB is rather on the low side.  For high speed it
amounts to about 1/3-second worth of data, which arguably is also a bit
low.  Increasing the default is easy enough, but the best choice isn't
obvious.

Alan Stern

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