Re: [PATCH v3] usb: ohci: Prevent missed ohci interrupts

[Guenter Roeck]

On 4/29/24 09:05, Alan Stern wrote:
> On Mon, Apr 29, 2024 at 08:40:10AM -0700, Guenter Roeck wrote:
> > Testing ohci functionality with qemu's pci-ohci emulation often results
> > in ohci interface stalls, resulting in hung task timeouts.
> >
> > The problem is caused by lost interrupts between the emulation and the
> > Linux kernel code. Additional interrupts raised while the ohci interrupt
> > handler in Linux is running and before the handler clears the interrupt
> > status are not handled. The fix for a similar problem in ehci suggests
> > that the problem is likely caused by edge-triggered MSI interrupts. See
> > commit 0b60557230ad ("usb: ehci: Prevent missed ehci interrupts with
> > edge-triggered MSI") for details.
> >
> > Ensure that the ohci interrupt code handles all pending interrupts before
> > returning to solve the problem.
> >
> > Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> > Cc: David Laight <David.Laight@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: 306c54d0edb6 ("usb: hcd: Try MSI interrupts on PCI devices")
> > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > ---
> > v3: Check if any interrupts are pending before reading intrenable
> >      Add 'Cc: stable@xxxxxxxxxxxxxxx'
> > v2: Only repeat if the interface is still active
> >
> > Note that I did not apply Alan's Reviewed-by: tag since I was not sure
> > if that was appropriate after the code change.
>
> I'm not too confident that the guess about all interrupt bits normally
> being off is correct, but in any case the extra test won't hurt.

OHCI_INTR_RHSC and OHCI_INTR_SF are often set but disabled while the system
starts, but afterwards it is mostly 0 during normal operation, i.e., while
there are no interface state changes.

> Reviewed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
>
> Guenter, if you want to work on a patch to cache the interrupt-enable
> value in the driver, feel free to do it.  I have too much other stuff
> going on to spend any significant time on ohci-hcd.

Unfortunately I am in the same situation. Also, I am not sure if doing that
would really be worth the trouble.

Guenter