linux-kernel - Re: [linux-usb-devel] USB performance bug since kernel 2.6.13 (CRITICAL???)

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Date:	Fri, 13 Oct 2006 17:08:46 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Open Source <opensource3141@...oo.com>
cc:	WolfgangMües <wolfgang@...w-muees.de>,
	<linux-usb-devel@...ts.sourceforge.net>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [linux-usb-devel] USB performance bug since kernel 2.6.13
 (CRITICAL???)

On Fri, 13 Oct 2006, Open Source wrote:

> Hi Wolfgang (and all),
> 
> Thanks for the input.  However, I am not understanding
> exactly why kernel mode is treated any differently than
> user mode for this sort of thing.  I am looking at the code
> in ehci-q.c and ehci-hcd.c.
> 
> It seems like the unlinking of completed URBs
> happens asynchronously on a timer.  This is a
> surprise to me since I thought this was happening
> on an IRQ from the host controller.  But if what I'm
> surmising is correct it would explain everything
> I am seeing.  I'm not able to ascertain how
> user mode drivers are treated differently than
> kernel mode drivers in this regard.  From what I
> can tell, all drivers would be broken equally!
> Can anyone who has more experience
> with this code confirm this for me?

I don't think so.  You must be mis-reading the code.  The only timers used 
in ehci-hcd are a couple of watchdogs; they shouldn't affect the normal 
URB completions which occur within ehci_work(), called by ehci_irq().

What Wolfgang meant was that user processes are subject to unpredictable 
delays from all kinds of sources.

Alan Stern

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