linux-kernel - Re: [RFC][PATCH 1/9] usb: hcd: Introduce usb_start/stop

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Date:	Wed, 18 Mar 2015 15:49:08 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Roger Quadros <rogerq@...com>
cc:	gregkh@...uxfoundation.org, <balbi@...com>,
	<dan.j.williams@...el.com>, <peter.chen@...escale.com>,
	<jun.li@...escale.com>, <mathias.nyman@...ux.intel.com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-omap@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/9] usb: hcd: Introduce usb_start/stop_hcd()

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@...com>

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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