linux-kernel - Re: [PATCH v2] usb: ohci: Prevent missed ohci interrupts

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Date: Mon, 29 Apr 2024 08:23:48 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: David Laight <David.Laight@...LAB.COM>, Gerd Hoffmann <kraxel@...hat.com>
Cc: Alan Stern <stern@...land.harvard.edu>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] usb: ohci: Prevent missed ohci interrupts

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