Re: [RFC][PATCH 1/9] usb: hcd: Introduce usb_start/stop_hcd()

From: Roger Quadros
Date: Thu Mar 19 2015 - 07:38:47 EST

On 18/03/15 21:49, Alan Stern wrote:
> 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.

Yes, good point.
> 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).

Right. This needs more work than I thought.
> 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.

So you mean directly calling usb_add/remove_hcd() from the OTG core?
I don't see any issues with that other than it being a heavy-duty operation
like you said and hope that it doesn't violate the OTG spec timing.

Looking at Figure 5-3: "HNP Sequence of Events (FS)" of the OTG 2.0 spec
we have about 150ms (X10) to switch from B-Device detected A connect (b_wait_acon state)
to driving bus reset (b_host state). I don't think this should be an issue in modern SoCs
but I'm not very sure.

In any case I can migrate to the add/remove hcd approach to simplify things.

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