gpiolib: cdev: Cleanup kfifo_out() error handling

The handling of kfifo_out() errors in read functions obscures any error.
The error condition should never occur but, while a ret is set to -EIO, it
is subsequently ignored and the read functions instead return the number
of bytes copied to that point, potentially masking the fact that any error
occurred.

Log a warning and return -EIO in the case of a kfifo_out() error to make
it clear something very odd is going on here.
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240529131953.195777-4-warthog618@gmail.comSigned-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpiolib-cdev.c

@@ -1642,16 +1642,15 @@ static ssize_t linereq_read(struct file *file, char __user *buf,
 					return ret;
 			}
 
-			ret = kfifo_out(&lr->events, &le, 1);
-		}
-		if (ret != 1) {
+			if (kfifo_out(&lr->events, &le, 1) != 1) {
 				/*
-			 * This should never happen - we were holding the
-			 * lock from the moment we learned the fifo is no
-			 * longer empty until now.
+				 * This should never happen - we hold the
+				 * lock from the moment we learned the fifo
+				 * is no longer empty until now.
 				 */
-			ret = -EIO;
-			break;
+				WARN(1, "failed to read from non-empty kfifo");
+				return -EIO;
+			}
 		}
 
 		if (copy_to_user(buf + bytes_read, &le, sizeof(le)))
@@ -1995,16 +1994,15 @@ static ssize_t lineevent_read(struct file *file, char __user *buf,
 					return ret;
 			}
 
-			ret = kfifo_out(&le->events, &ge, 1);
-		}
-		if (ret != 1) {
+			if (kfifo_out(&le->events, &ge, 1) != 1) {
 				/*
-			 * This should never happen - we were holding the lock
-			 * from the moment we learned the fifo is no longer
-			 * empty until now.
+				 * This should never happen - we hold the
+				 * lock from the moment we learned the fifo
+				 * is no longer empty until now.
 				 */
-			ret = -EIO;
-			break;
+				WARN(1, "failed to read from non-empty kfifo");
+				return -EIO;
+			}
 		}
 
 		if (copy_to_user(buf + bytes_read, &ge, ge_size))
@@ -2707,12 +2705,15 @@ static ssize_t lineinfo_watch_read(struct file *file, char __user *buf,
 			if (count < event_size)
 				return -EINVAL;
 #endif
-			ret = kfifo_out(&cdev->events, &event, 1);
+			if (kfifo_out(&cdev->events, &event, 1) != 1) {
+				/*
+				 * This should never happen - we hold the
+				 * lock from the moment we learned the fifo
+				 * is no longer empty until now.
+				 */
+				WARN(1, "failed to read from non-empty kfifo");
+				return -EIO;
 			}
-		if (ret != 1) {
-			ret = -EIO;
-			break;
-			/* We should never get here. See lineevent_read(). */
 		}
 
 #ifdef CONFIG_GPIO_CDEV_V1