[PATCH RESEND v2 2/2] usb: udc: Try to clarify an error message

[Miquel Raynal]

At some point when trying to use USB gadgets, two situations may arise
and lead to a failure. Either the UDC (USB Device Controller) is not
available at all (not described or not probed) or the UDC is already in
use. For instance, as the USB Ethernet gadget remains bound to the UDC,
the use of any other USB gadget (fastboot, dfu, etc) *after* will always
fail with the "couldn't find an available UDC" error.

Let's give a more helpful message by making a difference between the two
cases. Let's also hint people who would get this error and grep it into
the sources a better explanation of what's wrong with their workflow.

Signed-off-by: Miquel Raynal <miquel.raynal at bootlin.com>
---
While doing this I really wanted to add "much more" error comments but I
faced another reality: often the messages are there but use
pr_err/log_err which is actually silenced by default with LOGLEVEL=3, so
I consider this unnecessary, as decreasing the loglevel will make these
messages appear. I would have expected errors to be displayed, but I
understand it makes the binaries even bigger.

Resend: no change.

Changes in v2:
* s/UDC/UDCs/. I kept the sentence as it was as the suggested form did
  not sound well at all when there is only one UDC.
---
 drivers/usb/gadget/udc/udc-core.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/udc/udc-core.c b/drivers/usb/gadget/udc/udc-core.c
index 7f73926cb3e..8405b03462e 100644
--- a/drivers/usb/gadget/udc/udc-core.c
+++ b/drivers/usb/gadget/udc/udc-core.c
@@ -323,6 +323,7 @@ err1:
 int usb_gadget_probe_driver(struct usb_gadget_driver *driver)
 {
 	struct usb_udc		*udc = NULL;
+	unsigned int		udc_count = 0;
 	int			ret;
 
 	if (!driver || !driver->bind || !driver->setup)
@@ -330,12 +331,22 @@ int usb_gadget_probe_driver(struct usb_gadget_driver *driver)
 
 	mutex_lock(&udc_lock);
 	list_for_each_entry(udc, &udc_list, list) {
+		udc_count++;
+
 		/* For now we take the first one */
 		if (!udc->driver)
 			goto found;
 	}
 
-	printf("couldn't find an available UDC\n");
+	if (!udc_count)
+		printf("No UDC available in the system\n");
+	else
+		/* When this happens, users should 'unbind <class> <index>'
+		 * using the output of 'dm tree' and looking at the line right
+		 * after the USB peripheral/device controller.
+		 */
+		printf("All UDCs in use (%d available), use the unbind command\n",
+		       udc_count);
 	mutex_unlock(&udc_lock);
 	return -ENODEV;
 found:
-- 
2.34.1