Re: [PATCH v2] usb: ohci: Prevent missed ohci interrupts

[Guenter Roeck <linux@xxxxxxxxxxxx>]

On 4/27/24 14:00, David Laight wrote:
> From: Guenter Roeck
> > Sent: 24 April 2024 21:00
> >
> > 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>
> > Fixes: 306c54d0edb6 ("usb: hcd: Try MSI interrupts on PCI devices")
> > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > ---
> > v2: Only repeat if the interface is still active
> >
> >   drivers/usb/host/ohci-hcd.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/usb/host/ohci-hcd.c b/drivers/usb/host/ohci-hcd.c
> > index 4f9982ecfb58..bb6b50b4a356 100644
> > --- a/drivers/usb/host/ohci-hcd.c
> > +++ b/drivers/usb/host/ohci-hcd.c
> > @@ -888,6 +888,7 @@ static irqreturn_t ohci_irq (struct usb_hcd *hcd)
> >   	/* Check for an all 1's result which is a typical consequence
> >   	 * of dead, unclocked, or unplugged (CardBus...) devices
> >   	 */
> > +again:
> >   	if (ints == ~(u32)0) {
> >   		ohci->rh_state = OHCI_RH_HALTED;
> >   		ohci_dbg (ohci, "device removed!\n");
> > @@ -982,6 +983,13 @@ static irqreturn_t ohci_irq (struct usb_hcd *hcd)
> >   	}
> >   	spin_unlock(&ohci->lock);
> >
> > +	/* repeat until all enabled interrupts are handled */
> > +	if (ohci->rh_state != OHCI_RH_HALTED) {
> > +		ints = ohci_readl(ohci, &regs->intrstatus);
> > +		if (ints & ohci_readl(ohci, &regs->intrenable))
>
> Doesn't the driver know which interrupts are enabled?
> So it should be able to avoid doing two (likely) slow io reads?
> (PCIe reads are pretty much guaranteed to be high latency.)

No, the driver does not cache intrenable.

Guenter