Subject: Re: 2.5.69 Interrupt Latency
From: Paul Fulghum <paulkf@microgate.com>
To: Andrew Morton <akpm@digeo.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       Arnd Bergmann <arnd@arndb.de>, johannes@erdfelt.com
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Date: 12 May 2003 09:06:09 -0500
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On Mon, 2003-05-12 at 08:57, Paul Fulghum wrote:
> On Fri, 2003-05-09 at 16:28, Andrew Morton wrote:
> 
> > This code was added to wakeup_hc().  It is called from uhci_irq():
> > 
> > +	/* Global resume for 20ms */
> > +	outw(USBCMD_FGR | USBCMD_EGSM, io_addr + USBCMD);
> > +	wait_ms(20);
> > 
> > The changelog just says "Minor patch for uhci-hcd.c"
> > 
> > Can you delete the wait_ms() and see if that is our culprit?
> 
> This is the culprit.
> 
> Removing this line corrects the latency problems on
> the server. A 20ms delay seems pretty excessive for an
> interrupt handler. I'm not sure what it is supposed to
> accomplish, but this seems like something that should
> be scheduled to run outside of the ISR.
> 
> I must have messed up a test on the laptop that is
> also showing latency problems. On the laptop the
> problem *is* in both 2.5.68/2.5.69 and *is not*
> eliminated by turning off USB. The laptop uses the
> ohci driver anyways which is not effected by this patch.
> The laptop does not show latency problems on 2.4.20.
> 
> So the patch above is definately a problem,
> but the problem I am seeing on the laptop
> is something unrelated, but part of 2.5.x
> (which I will investigate further).
> 
> Thanks,
> Paul

I forgot to add this snippet from the /var/log/messages file
of the server in case it helps the USB maintainer
in evaluating what to do about the above patch.

kernel: drivers/usb/host/uhci-hcd.c: USB Universal Host Controller Interface driver v2.0
kernel: uhci-hcd 00:04.2: Intel Corp. 82371AB/EB/MB PIIX4 
kernel: uhci-hcd 00:04.2: irq 19, io base 0000fce0
kernel: Please use the 'usbfs' filetype instead, the 'usbdevfs' name is deprecated.
kernel: uhci-hcd 00:04.2: new USB bus registered, assigned bus number 1
kernel: hub 1-0:0: USB hub found
kernel: hub 1-0:0: 2 ports detected


-- 
Paul Fulghum, paulkf@microgate.com
Microgate Corporation, http://www.microgate.com


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