usb: gadget: composite: Allow bMaxPower=0 if self-powered

Currently the composite driver encodes the MaxPower field of
the configuration descriptor by reading the c->MaxPower of the
usb_configuration only if it is non-zero, otherwise it falls back
to using the value hard-coded in CONFIG_USB_GADGET_VBUS_DRAW.
However, there are cases when a configuration must explicitly set
bMaxPower to 0, particularly if its bmAttributes also has the
Self-Powered bit set, which is a valid combination.

This is specifically called out in the USB PD specification section
9.1, in which a PDUSB device "shall report zero in the bMaxPower
field after negotiating a mutually agreeable Contract", and also
verified by the USB Type-C Functional Test TD.4.10.2 Sink Power
Precedence Test.

The fix allows the c->MaxPower to be used for encoding the bMaxPower
even if it is 0, if the self-powered bit is also set.  An example
usage of this would be for a ConfigFS gadget to be dynamically
updated by userspace when the Type-C connection is determined to be
operating in Power Delivery mode.
Co-developed-by: Ronak Vijay Raheja <rraheja@codeaurora.org>
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Ronak Vijay Raheja <rraheja@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Link: https://lore.kernel.org/r/20210720080907.30292-1-jackp@codeaurora.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/composite.c

@@ -482,7 +482,7 @@ static u8 encode_bMaxPower(enum usb_device_speed speed,
 {
 	unsigned val;
 
-	if (c->MaxPower)
+	if (c->MaxPower || (c->bmAttributes & USB_CONFIG_ATT_SELFPOWER))
 		val = c->MaxPower;
 	else
 		val = CONFIG_USB_GADGET_VBUS_DRAW;
@@ -936,7 +936,11 @@ static int set_config(struct usb_composite_dev *cdev,
 	}
 
 	/* when we return, be sure our power usage is valid */
-	power = c->MaxPower ? c->MaxPower : CONFIG_USB_GADGET_VBUS_DRAW;
+	if (c->MaxPower || (c->bmAttributes & USB_CONFIG_ATT_SELFPOWER))
+		power = c->MaxPower;
+	else
+		power = CONFIG_USB_GADGET_VBUS_DRAW;
+
 	if (gadget->speed < USB_SPEED_SUPER)
 		power = min(power, 500U);
 	else