usb: gadget: core: change the comment for usb_gadget_connect

The pullup does not need to be enabled at below situations:
- For platforms which the hardware pullup starts after setting
register even they do not see the VBUS. If the pullup is always
enabled for these platforms, it will consume more power and
break the USB IF compliance tests [1].
- For platforms which need to do BC 1.2 charger detection after
seeing the VBUS. Pullup D+ will break the charger detection
flow.
- For platforms which the system suspend is allowed when the
connection with host is there but the bus is not at suspend.
For these platforms, it is better to disable pullup when
entering the system suspend otherwise the host may confuse
the device behavior after controller is in low power mode.
Disable pullup is considered as a disconnection event from
host side.

[1] USB-IF Full and Low Speed Compliance Test Procedure
F Back-voltage Testing
Section 7.2.1 of the USB specification requires that no device
shall supply (source) current on VBUS at its upstream facing
port at any time. From VBUS on its upstream facing port,
a device may only draw (sink) current. They may not provide power
to the pull-up resistor on D+/D- unless VBUS is present (see
Section 7.1.5).
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Link: https://lore.kernel.org/r/20201230051114.21370-1-peter.chen@nxp.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/core.c

@@ -659,8 +659,7 @@ EXPORT_SYMBOL_GPL(usb_gadget_vbus_disconnect);
  *
  * Enables the D+ (or potentially D-) pullup.  The host will start
  * enumerating this gadget when the pullup is active and a VBUS session
- * is active (the link is powered).  This pullup is always enabled unless
- * usb_gadget_disconnect() has been used to disable it.
+ * is active (the link is powered).
  *
  * Returns zero on success, else negative errno.
  */