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Date: Mon, 29 Apr 2024 06:34:00 -0700
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Subject: Re: [PATCH v2] usb: ohci: Prevent missed ohci interrupts
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: David Laight <David.Laight@aculab.com>,
 Alan Stern <stern@rowland.harvard.edu>,
 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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From: Guenter Roeck <linux@roeck-us.net>
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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