Re: [PATCH] usb: echi-hcd: Add register access check in shutdown

[Srinivas Kandagatla]

On 18/05/16 15:56, Alan Stern wrote:
> On Wed, 18 May 2016, Srinivas Kandagatla wrote:
>
> > This patch adds a check in ehci_shutdown(), to make sure
> > that the register access is available before accessing registers.
> >
> > The use case is simple, for boards like DB410c where the usb host
> > or device functionality is decided based on the micro-usb cable
> > presence. If the board boots up with micro-usb connected and the
> > host driver is probed, but the ehci_setup() has not been done yet,
> > then a system shutdown would trigger below NULL pointer exception
> > without this patch.
>
> How can that happen?  While the host driver is probed, the probing
> thread holds the device lock.  But the system shutdown routine acquires
> the device lock before invoking the ->shutdown callback.  Therefore the
> two things cannot happen concurrently.

No, I did not mean them happening concurrently, I mean that the host 
driver is up, however ehci_setup() is not done yet.

Will change the comments if its misleading the reader.

> > Unable to handle kernel NULL pointer dereference at virtual address
> > 00000008
>
> ...
>
> > --- a/drivers/usb/host/ehci-hcd.c
> > +++ b/drivers/usb/host/ehci-hcd.c
> > @@ -368,6 +368,9 @@ static void ehci_shutdown(struct usb_hcd *hcd)
> >   {
> >   	struct ehci_hcd	*ehci = hcd_to_ehci(hcd);
> >
> > +	if (!HCD_HW_ACCESSIBLE(hcd))
> > +		return;
> > +
> >   	spin_lock_irq(&ehci->lock);
> >   	ehci->shutdown = true;
> >   	ehci->rh_state = EHCI_RH_STOPPING;
>
> This doesn't seem like the right place.  What you really should do is
> skip calling ehci_silence_controller() if the hardware isn't
> accessible.  That's where the hardware gets touched, not in
> ehci_shutdown().
Yep , that should work as well.

Will send a v2 patch.

thanks,
srini

> Alan Stern