Re: XT-PIC interrupts blocked by usbserial ? [Was Re: Intel ICH9M bug : sata unusable with usbserial]

[Greg KH <greg@kroah.com>]

On Thu, Mar 03, 2011 at 01:10:10PM -0500, Mark Lord wrote:
> What is most likely happening here, is that the USB interrupt handler
> is returning IRQ_HANDLED under some condition where IRQ_NONE is required.
> 
> In other words, USB thought it got an interrupt, but didn't really,
> but since it returns IRQ_HANDLED regardless, then other
> device drivers sharing that IRQ don't necessarily get called.
> If this happens repeatedly, then it could disrupt the other devices.

I doubt this is the issue, as we would have seen it long before now.

> Now, that's not overly likely, but then again it's probably not
> well tested with everyone using APICs nowadays.
> 
> Or, perhaps the PL2303 driver leaves an interrupt active/pending
> without clearing it in hardware, so the interrupt keeps recurring
> over and over.  This (unlikely) is easy to check for, by watching
> the /proc/interrupts count for the USB on your APIC-enabled kernel.

A usb driver has no interrupt handling at all, so it just flat out can
not do this.  It's up to the usb host controller driver, that is what
handles interrupts.

> If the numbers increase at a phenomenal rate, then there might
> be an issue.  With an active GPS, one would expect the numbers to
> be incrementing steadily, say a few dozen or a hundred or so per second.

It all depends on the traffic to the USB device and how well behaved it
is, as to how many interrupts it generates.

thanks,

greg k-h