kernel: fix error propagation for device deferred initialization

This fix makes sure that do_device_init() returns a negative value if
the device's initialization failed. Previously, it mistakely returned
+errno instead of -errno.

This oversight happened during the refactoring of z_sys_init_run_level()
to support deferred initialization, from which most of do_device_init()
code derives. The rc value computed and stored in dev->state->init_res
is the POSITIVE value of the resulting errno. Returning rc therefore
breaks the convention of a negative value to signal failure.

Signed-off-by: Loic Domaigne <tech@domaigne.com>

Author: ljd42

kernel/init.c

@@ -313,28 +313,43 @@ static int do_device_init(const struct device *dev)
 
 	if (dev->ops.init != NULL) {
 		rc = dev->ops.init(dev);
-		/* Mark device initialized. If initialization
-		 * failed, record the error condition.
+		/* If initialization failed, record in dev->state->init_res
+		 * the POSITIVE value of the resulting errno
 		 */
 		if (rc != 0) {
+			/* device's init function should return:
+			 *   0 on success
+			 *   a negative value on failure (-errno)
+			 * errno value maps to an uint8_t range as of now.
+			 */
+			__ASSERT(rc >= -UINT8_MAX && rc < 0, "device %s init: invalid error (%d)",
+				 dev->name, rc);
+
 			if (rc < 0) {
 				rc = -rc;
 			}
+			/* handle error value overflow in production
+			 * this is likely a bug in the device's init function. Signals it
+			 */
 			if (rc > UINT8_MAX) {
 				rc = UINT8_MAX;
 			}
 			dev->state->init_res = rc;
 		}
 	}
 
+	/* device initialization has been invoked */
 	dev->state->initialized = true;
 
 	if (rc == 0) {
 		/* Run automatic device runtime enablement */
 		(void)pm_device_runtime_auto_enable(dev);
 	}
 
-	return rc;
+	/* here, the value of rc is either 0 or +errno
+	 * flip the sign to return a negative value on failure as expected
+	 */
+	return -rc;
 }
 
 /**