linux-kernel - Re: [PATCH v3] usb: ohci: Prevent missed ohci interrupts

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Date: Mon, 29 Apr 2024 09:40:38 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
 David Laight <David.Laight@...lab.com>, Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [PATCH v3] usb: ohci: Prevent missed ohci interrupts

On 4/29/24 09:05, Alan Stern wrote:
> On Mon, Apr 29, 2024 at 08:40:10AM -0700, Guenter Roeck wrote:
>> 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@...hat.com>
>> Cc: David Laight <David.Laight@...lab.com>
>> Cc: stable@...r.kernel.org
>> Fixes: 306c54d0edb6 ("usb: hcd: Try MSI interrupts on PCI devices")
>> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>> ---
>> v3: Check if any interrupts are pending before reading intrenable
>>      Add 'Cc: stable@...r.kernel.org'
>> v2: Only repeat if the interface is still active
>>
>> Note that I did not apply Alan's Reviewed-by: tag since I was not sure
>> if that was appropriate after the code change.
> 
> I'm not too confident that the guess about all interrupt bits normally
> being off is correct, but in any case the extra test won't hurt.
> 

OHCI_INTR_RHSC and OHCI_INTR_SF are often set but disabled while the system
starts, but afterwards it is mostly 0 during normal operation, i.e., while
there are no interface state changes.

> Reviewed-by: Alan Stern <stern@...land.harvard.edu>
> 
> Guenter, if you want to work on a patch to cache the interrupt-enable
> value in the driver, feel free to do it.  I have too much other stuff
> going on to spend any significant time on ohci-hcd.
> 

Unfortunately I am in the same situation. Also, I am not sure if doing that
would really be worth the trouble.

Guenter