Intel Core Duo USB Issue: A Mischaracterized Bug

Problem #2 - Disabling a USB device doesn't work

If we now know that this problem should affect both Napa (Core Duo) and Sonoma (Pentium M) platforms, why is it that the results that we've seen to date don't support that theory?  While we can only comment on tests that we've run ourselves, we did run into some issues with the ASUS W5F/W5A notebooks, which are quickly becoming popular Napa/Sonoma test platforms. 

The reason that the two ASUS notebooks are nice to use is because they are virtually identical systems, one simply uses a Sonoma motherboard with a Pentium M CPU while the other uses a Napa board with a Core Duo CPU.  In the end, it provides an excellent level playing field for comparing Core Duo to its predecessor. 

The problem is that both ASUS notebooks feature an integrated USB 2.0 camera, which we originally didn't expect to be a problem because we left it disabled and without its drivers installed.  After all, if a device doesn't have its driver initialized, it shouldn't be impacting our test results, correct?  But let's look at the Microsoft KB article a little closer. Under the cause of the problem, the article states that "Windows XP SP2 installs a USB 2.0 driver that initializes any connected USB device."  It sounds like simply connecting a USB 2.0 device, even if you don't load the drivers for it, would still trigger the bug.  But does that mean if you disable the device, the asynchronous scheduler is still running?  There's only one way to find out, so we ran the Mobile Mark test again, this time on a Lenovo T60 with no integrated USB 2.0 devices. We then connected a USB 2.0 device to it, but did not install a driver, as well as tried disabling the device:

Lenovo T60	Nothing Connected	USB 2.0 Device Connected, No Driver Installed	USB Device Disabled
Reader 2002SE Battery Life in Minutes	286	242	235

With a USB 2.0 device (an external TV tuner) connected, but no driver installed, the battery life on our T60 dropped from 286 minutes down to 242, a reduction of 15.3%.  Disabling the device had no effect either, as we recorded a battery life of 235 minutes.  These findings are particularly important because both ASUS notebooks, the Sonoma and the Napa, feature an integrated USB 2.0 camera.  Even without the drivers loaded, short of ripping the camera out of the system, these two notebooks are terrible reference points for the USB power draw issue as both of them exhibit the issue without even plugging in any external devices! 

To confirm, we looked at C3 time once more on both platforms:

The ASUS notebooks never enter C3 or lower power states, even with no external devices connected

Just as we suspected, the CPUs aren't allowed into their lower power states due to the integrated USB 2.0 camera. 

To further confirm, we applied the workaround documented in the Microsoft KB article.  With no external devices connected to either system, their battery life jumped by approximately another hour:

Reader 2002SE Battery Life in Minutes	Nothing Connected	Nothing Connected (MS Fix Applied)
ASUS W5F (Napa/Core Duo)	219	264
ASUS W5A (Sonoma/Pentium M)	204	273

While this means that our battery life tests from our Core Duo notebook article are significantly lower than they should be (we'll be providing a follow-up to that article in the near future), it also means that neither ASUS notebook should be used in pinpointing the cause or effects of this USB problem. 

Adding an external USB 2.0 device to either ASUS notebook does result in an additional drop in battery life, but the initial damage is done by the integrated camera that you can't unplug.  To truly isolate this problem, we'll need two notebooks that don't have any integrated USB 2.0 devices, which is why we turned to the Lenovo Thinkpad T43 and T60.