Re: Calibration issues with USB disc present.

[john stultz <johnstul@us.ibm.com>]

On Fri, 2005-11-11 at 13:37 -0800, George Anzinger wrote:
> John,
> 
> Have you run into this.  One of the USB disc controllers has the ability to boot the system, 
> however, it needs SMM code to do this.  This SMM code, somehow, causes SMI interrupts (which are 
> higher priority than NMI interrutps and not maskable) which it needs to do its thing.
> 
> Problem is that if one of these occurs while calibrating the TSC or the delay code, it can cause a 
> wrong result.  We have seen both a too long and a too short result (depending on where the interrut 
> happens).
> 
> They have found the root cause of TSC calibration problem.
> Now they ask for the fix or workaround.
> 
> That is the BIOS is periodically interrupted by USB controller and the CPU
> waits during the processing of these interrupts.
> Their experiments say the interrupt interval is 260mSec and the BIOS needs
> 150uSec - 200uSec for processing.
> It is proved that the problem doesn't reproduce by masking such SMI in BIOS.
> They say SMI is for BIOS emulation for connecting legacy devices to USB.
> Without such an emulation it's impossible to boot from USB-FD for instance,
> they say too.

Hmmm. I haven't heard of this issue specifically, but yes, I'm quite
familiar with the pain BIOS SMIs can cause and I'm not surprised that it
would affect the TSC/delay calibration code.

Is this still an issue w/ 2.6.14? I know the new TSC based delay
calibration code is supposed to be SMI resilient, but I haven't really
played with it closely.

Not sure what the best method to move forward would be. I suspect
disabling the SMI code early in boot (I thought the usb legacy handoff
stuff already did this?) would help. Then the actual Linux USB drivers
can take over before we switch from the initrd to the root filesystem.

Greg, do you have a suggestion?

thanks
-john

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