Re: [Qemu-devel] [PATCH 2/2] ohci: clear pending SOF on suspend

[Thomas Huth <thuth@redhat.com>]

On 16/12/15 14:39, Laurent Vivier wrote:
> On overcommitted CPU, kernel can be so slow that an interrupt can
> be triggered by the device whereas the driver is not ready to receive
> it. This drives us into an infinite loop.
> 
> On suspend, if a SOF interrupt is raised between the stop of the
> device processing and the change of the device internal state to
> OHCI_USB_SUSPEND (QEMU stops SOF timer on this state change), this
> interrupt is never acknowledged.
> 
> This patch clears pending SOF interrupt on OHCI_USB_SUSPEND setting.
> 
> Some details:
> 
> - ohci_irq(): the OHCI interrupt handler, acknowledges the SOF IRQ
>   only if the state of the driver (rh_state) is OHCI_STATE_RUNNING.
>   So if this interrupt happens and the driver is not in this state,
>   the function is called again and again, moving the system to a
>   CPU starvation.
> 
> - ohci_rh_suspend(): the function stop the operation and acknowledge
>   pending interrupts (but doesn't disable it). Later in the function,
>   the device is moved to OHCI_SUSPEND_STATE, and the driver to
>   OHCI_RH_SUSPENDED. If between the moment when the interrupt is
>   acknowledged and the moment when the device is suspended a new
>   interrupt is raised, it will be never acknowledged because the
>   driver is now not in OHCI_RH_RUNNING state.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  hw/usb/hcd-ohci.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/hw/usb/hcd-ohci.c b/hw/usb/hcd-ohci.c
> index 5f15ebb..b5a4e39 100644
> --- a/hw/usb/hcd-ohci.c
> +++ b/hw/usb/hcd-ohci.c
> @@ -1438,6 +1438,9 @@ static void ohci_set_ctl(OHCIState *ohci, uint32_t val)
>          break;
>      case OHCI_USB_SUSPEND:
>          ohci_bus_stop(ohci);
> +        /* clear pending SF otherwise driver loops in ohci_irq() */

May I suggest to talk about "Linux driver" instead of only "driver"
here? ... QEMU also supports other guests, so the context might not be
clear otherwise.

> +        ohci->intr_status &= ~OHCI_INTR_SF;
> +        ohci_intr_update(ohci);
>          break;
>      case OHCI_USB_RESUME:
>          trace_usb_ohci_resume(ohci->name);

Apart from that nit in the comment, patch looks sane to me.

Reviewed-by: Thomas Huth <thuth@redhat.com>