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

[Guenter Roeck <linux@roeck-us.net>]

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@redhat.com>
>> Fixes: 306c54d0edb6 ("usb: hcd: Try MSI interrupts on PCI devices")
>> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
>> ---
>> 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