Re: USB interrupt handler routine

[Alan Stern]

On Sat, 13 Dec 2008, Greg KH wrote:

> <added linux-usb list, that's the proper place for it...)
> 
> On Thu, Dec 11, 2008 at 08:35:37PM -0800, Keith Packard wrote:
> > We recently made a patch in the Intel DRM driver:
> > 
> > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b60678a75d44fa9d5969f79781bd856ad5858609
> > 
> > This switches from non-MSI to MSI-mode.
> > 
> > It "should" have had no effect, but our concern is that what we're
> > really seeing is interrupt sharing troubles. We've had several people
> > say that they got an interrupt flood when plugging in a USB stick or
> > external disk.

Which drivers are using the IRQ that gets flooded?  Can we see a dmesg 
log from a kernel built with CONFIG_USB_DEBUG enabled?

> > Switching the graphics driver to MSI mode "cures" the
> > bug, but that sure seems like a work-around rather than a bug fix to me.
> > 
> > When our driver is active, it can generate a lot of interrupts,
> > potentially thousands per second. Is it possible that the UHCI or EHCI
> > drivers could get confused if checking for interrupt status this often?

I don't think so.

> > Eric noted that the USB driver

Which USB driver?  Does this refer to usb_hcd_irq()?

> > appears to not check and ACK interrupt
> > status unconditionally, preferring to check the software state
> > beforehand. I'm wondering if this may open up a potential race between
> > hardware state change and ISR bit setting.

Not if the drivers are written correctly.  As far as I know, they are.

Alan Stern

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