On 18/05/16 17:15, Alan Stern wrote:
On Wed, 18 May 2016, Srinivas Kandagatla wrote:
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.
I don't understand. ehci_setup() is called as part of the probe
procedure. How can the host driver be up if ehci_setup() is not done
yet?
Yes, this is true in ehci-msm driver, The driver does not add usb host
by default in probe when phy is otg capable.
The usb host is added dynamically by the msm_otg driver depending on the
the micro USB cable plug/un-plug events via extcon.
Are you saying that when the system is plugged into the "B" end of an
OTG cable, ehci_setup() doesn't get called at all?
Yes, for echi-msm driver, not sure about other host controller drivers.
And would the same thing happen if the system started out as the host
but then used HNP to change into the peripheral?
I don't think so, As the ehci->regs get populated once we enter the
ehci_setup(), so ehci_halt() will never get chance to dereference null
in this case.
Fault occurs only if the driver did not enter into host mode and system
reboot/shutdown is requested.
--srini
Alan Stern
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