Reported by Nikolaus Schaller, Sep 9, 2014

currently, both drivers are coupled by a private function call which 
makes it impossible to link if they are configured as modules.

This is not mainline code but introduced by our GTA04 stuff.

The code is also marked as "hack":


static ssize_t twl4030_usb_vbus_out_store(struct device *dev,
		struct device_attribute *attr,
		const char *buf, size_t n)
{
	struct twl4030_usb *twl = dev_get_drvdata(dev);
	/* hack, should probably not be called directly this way */
	extern void twl4030_charger_enable_usb_ext(bool);

	if (sysfs_streq(buf, "on")) {
		twl4030_charger_enable_usb_ext(0);
		twl4030_i2c_access(twl, 1);
		twl4030_usb_set_bits(twl, ULPI_OTG_CTRL, ULPI_OTG_CTRL_DRVVBUS);
		twl4030_i2c_access(twl, 0);
	} else if (sysfs_streq(buf, "off")) {
		twl4030_i2c_access(twl, 1);
		twl4030_usb_clear_bits(twl, ULPI_OTG_CTRL,
			ULPI_OTG_CTRL_DRVVBUS);
		twl4030_i2c_access(twl, 0);
		twl4030_charger_enable_usb_ext(1);
	} else
		return -EINVAL;
	return n;
}

As far as I understand this code it disables charging if the OTG 
VBUS driver is turned on.

But it also appears that twl4030_usb_vbus_out_store() is only called 
if someone writes "on" or "off" to 
/sys/.../vbus_out

So we can simply ask the user space that writes "on" to 
disable the charger...

On the other hand the twl4030_charger listens to some 
usb_notifier_block and passes an event to its work procedure in 
twl4030_bci_usb_ncb().

So there is some communication path between the usb (phy) and the 
charger. We just need to create and send the correct 
notification_block in drivers/phy/phy-twl4030-usb.

The events we should send are:
USB_EVENT_VBUS / USB_EVENT_CHARGER or USB_EVENT_NONE.

They are usually sent by: --- hm good question. I did not find any 
code that sends something using the USB_EVENT_CHARGER constant?!?