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

[Guenter Roeck <linux@xxxxxxxxxxxx>]

On 4/28/24 23:49, Gerd Hoffmann wrote:
>    Hi,
>
> > > > +	/* 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.
>
> Does the driver ever change intrenable after initialization?

$ git grep -e intrenable -e intrdisable drivers/usb/host/*ohci*c | grep ohci_writel
drivers/usb/host/ohci-hcd.c:	ohci_writel(ohci, (u32) ~0, &ohci->regs->intrdisable);
drivers/usb/host/ohci-hcd.c:		ohci_writel (ohci, OHCI_INTR_OC, &ohci->regs->intrenable);
drivers/usb/host/ohci-hcd.c:	ohci_writel (ohci, OHCI_INTR_MIE, &ohci->regs->intrdisable);
drivers/usb/host/ohci-hcd.c:	ohci_writel (ohci, mask, &ohci->regs->intrenable);
drivers/usb/host/ohci-hcd.c:			ohci_writel (ohci, OHCI_INTR_UE, &regs->intrdisable);
drivers/usb/host/ohci-hcd.c:		ohci_writel(ohci, OHCI_INTR_RHSC, &regs->intrdisable);
drivers/usb/host/ohci-hcd.c:		ohci_writel (ohci, OHCI_INTR_SF, &regs->intrdisable);
drivers/usb/host/ohci-hcd.c:		ohci_writel (ohci, OHCI_INTR_MIE, &regs->intrenable);
drivers/usb/host/ohci-hcd.c:	ohci_writel (ohci, OHCI_INTR_MIE, &ohci->regs->intrdisable);
drivers/usb/host/ohci-hcd.c:	ohci_writel(ohci, OHCI_INTR_MIE, &ohci->regs->intrdisable);
drivers/usb/host/ohci-hcd.c:		ohci_writel(ohci, OHCI_INTR_MIE, &ohci->regs->intrenable);
drivers/usb/host/ohci-hub.c:	ohci_writel(ohci, OHCI_INTR_SF, &ohci->regs->intrdisable);
drivers/usb/host/ohci-hub.c:	ohci_writel (ohci, OHCI_INTR_INIT, &ohci->regs->intrenable);
drivers/usb/host/ohci-hub.c:		ohci_writel (ohci, OHCI_INTR_SF, &ohci->regs->intrenable);
drivers/usb/host/ohci-hub.c:			ohci_writel(ohci, rhsc_enable, &ohci->regs->intrenable);
drivers/usb/host/ohci-hub.c:	ohci_writel(ohci, OHCI_INTR_RHSC, &ohci->regs->intrenable);
drivers/usb/host/ohci-q.c:	ohci_writel (ohci, OHCI_INTR_SF, &ohci->regs->intrenable);

> PCIe reads are expensive, especially in virtual machines where this
> goes vmexit to qemu, so doing that for a piece of information the
> driver should have (or is able to calculate) should indeed better
> be avoided.

I would agree, but I really think that should be a separate patch
if implemented.

Guenter