dwc2: remote wakeup implementation

[Maxim Devaev <mdevaev@gmail.com>]

Hello.

The dwc3 driver in gadget mode has the ability to send
a remote wakeup signal to the host by writing 1 to the srp file:

echo 1 > /sys/class/udc/XXX/srp

I am trying to implement the same feature for dwc2, but I ran
into a problem. My naive and clumsy implementation is able
to wake up the host, but after that dwc2 stops processing
any interrupts and the gadget stops working.

At the same time, if I send the host to suspend, and then
it is awakened independently (for example, using the hardware
keyboard), then I see that dwc2 handles interrupts
and starts working again. I used the protocol analyzer
and made sure that the gadget with dwc2 is really correctly
suspended and wakes up.

I suppose remote wakeup signaling causes the controller
to switch to some incomprehensible (for me) state, and I need
to do something so that everything works again and dwc2
can receive a wake-up event from the host after signaling.

I don't have any DesignWare documentation, so I hope someone
can help me. Thanks.

diff --git a/drivers/usb/dwc2/gadget.c b/drivers/usb/dwc2/gadget.c
index c581ee41a..14dfb678a 100644
--- a/drivers/usb/dwc2/gadget.c
+++ b/drivers/usb/dwc2/gadget.c
@@ -4660,6 +4660,21 @@ static int dwc2_hsotg_vbus_draw(struct usb_gadget *gadget, unsigned int mA)
 	return usb_phy_set_power(hsotg->uphy, mA);
 }
 
+static int dwc2_hsotg_wakeup(struct usb_gadget *gadget)
+{
+	struct dwc2_hsotg *hsotg = to_hsotg(gadget);
+	unsigned long flags;
+
+	spin_lock_irqsave(&hsotg->lock, flags);
+
+	dwc2_set_bit(hsotg, DCTL, DCTL_RMTWKUPSIG);
+	mdelay(10);
+	dwc2_clear_bit(hsotg, DCTL, DCTL_RMTWKUPSIG);
+
+	spin_unlock_irqrestore(&hsotg->lock, flags);
+	return 0;
+}
+
 static const struct usb_gadget_ops dwc2_hsotg_gadget_ops = {
 	.get_frame	= dwc2_hsotg_gadget_getframe,
 	.set_selfpowered	= dwc2_hsotg_set_selfpowered,
@@ -4668,6 +4683,7 @@ static const struct usb_gadget_ops dwc2_hsotg_gadget_ops = {
 	.pullup                 = dwc2_hsotg_pullup,
 	.vbus_session		= dwc2_hsotg_vbus_session,
 	.vbus_draw		= dwc2_hsotg_vbus_draw,
+	.wakeup			= dwc2_hsotg_wakeup,
 };
 
 /**