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Date: Wed, 24 Apr 2024 14:15:39 -0400
From: Alan Stern <stern@rowland.harvard.edu>
To: Guenter Roeck <linux@roeck-us.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
  linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
  Gerd Hoffmann <kraxel@redhat.com>
Subject: Re: [PATCH] usb: ohci: Prevent missed ohci interrupts
Message-ID: <57511487-e622-476d-adb2-5dfc725032d4@rowland.harvard.edu>
References: <20240424170250.3738037-1-linux@roeck-us.net>
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On Wed, Apr 24, 2024 at 10:02:50AM -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@redhat.com>
> Fixes: 306c54d0edb6 ("usb: hcd: Try MSI interrupts on PCI devices")
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> ---
>  drivers/usb/host/ohci-hcd.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/usb/host/ohci-hcd.c b/drivers/usb/host/ohci-hcd.c
> index 4f9982ecfb58..4d764eb6c1e5 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 +983,11 @@ static irqreturn_t ohci_irq (struct usb_hcd *hcd)
>  	}
>  	spin_unlock(&ohci->lock);
>  
> +	/* repeat until all enabled interrupts are handled */
> +	ints = ohci_readl(ohci, &regs->intrstatus);
> +	if (ints & ohci_readl(ohci, &regs->intrenable))
> +		goto again;

If we take the repeat, we don't want to return IRQ_NOTMINE by accident.  To 
prevent this, we should check that ohci->rh_state != OHCI_RH_HALTED before 
re-reading ints and jumping back.

(If rh_state _is_ OHCI_RH_HALTED, it means the device is supposedly stopped 
and disabled for generating further interrupt requests, so we shouldn't 
need to worry about any outstanding intrstatus bits still set.)

Alan Stern

> +
>  	return IRQ_HANDLED;
>  }
>  
> -- 
> 2.39.2
>