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

[Guenter Roeck <linux@xxxxxxxxxxxx>]

On 4/29/24 07:01, David Laight wrote:
> From: Guenter Roeck
> > Sent: 29 April 2024 14:34
> >
> > 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);
>
> At least the hardware has separate enable/disable registers
> so the driver isn't doing RMW cycles.
>
> I'd guess that the normal condition is that no interrupts are pending.
> So it can be held to one (slow) read by checking:
> 	if (ints && (ints & ohci_readl(ohci, &regs->intrenable)))

Guess that can't hurt. I'll send v3.

Thanks,
Guenter