On Wed, 18 Mar 2015, Roger Quadros wrote:
> To support OTG we want a mechanism to start and stop
> the HCD from the OTG state machine. Add usb_start_hcd()
> and usb_stop_hcd().
>
> Signed-off-by: Roger Quadros <rogerq@xxxxxx>
There are a few problems in this proposed patch.
> +int usb_start_hcd(struct usb_hcd *hcd)
> +{
> + int retval;
> + struct usb_device *rhdev = hcd->self.root_hub;
> +
> + if (hcd->state != HC_STATE_HALT) {
> + dev_err(hcd->self.controller, "not starting a running HCD\n");
> + return -EINVAL;
> + }
> +
> + hcd->state = HC_STATE_RUNNING;
> + retval = hcd->driver->start(hcd);
> + if (retval < 0) {
> + dev_err(hcd->self.controller, "startup error %d\n", retval);
> + hcd->state = HC_STATE_HALT;
> + return retval;
> + }
> +
> + /* starting here, usbcore will pay attention to this root hub */
> + if ((retval = register_root_hub(hcd)) != 0)
> + goto err_register_root_hub;
If the host controller is started more than once, you will end up
unregistering and re-registering the root hub. The device core does
not allow this. Once a device has been unregistered, you must not try
to register it again -- you have to allocate a new device and register
it instead.
Also, although you call the driver's ->start method multiple times, the
->reset method is called only once, when the controller is first
probed. It's not clear that this will work in a situation where the HC
and the UDC share hardware state; after the UDC is stopped it may be
necessary to reset the HC before it can run again.
It might be possible to make this work, but I suspect quite a few
drivers would need rewriting first. As another example of the problems
you face, consider how stopping a host controller will interact with
the driver's PM support (both system suspend and runtime suspend).
It would be a lot simpler to unbind the host controller driver
completely when switching to device mode and rebind it when switching
back. I guess that is the sort of heavy-duty approach you want to
avoid, but it may be the only practical way forward.
Alan Stern
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