spi: fix the read path in spidev

This got broken by the recent "fix rmmod $spi_driver while spidev-user is
active".  I tested the rmmod & write path but didn't check the read path.
I am sorry.  The read logic changed and spidev_sync_read() +
spidev_sync_write() do not return zero on success anymore but the number
of bytes that has been transfered over the bus.  This patch changes the
logic and copy_to_user() gets called again.

The write path returns the number of bytes which are written to the
underlying device what may be less than the requested size.  This patch
makes the same change to the read path or else we request a read of 20
bytes, get 10, don't call copy to user and report to the user that we read
10 bytes.

[[email protected]: remove test of known-to-be-zero local]
Signed-off-by: Sebastian Siewior <[email protected]>
Acked-by: David Brownell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/spi/spidev.c

@@ -167,14 +167,14 @@ spidev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
 
 	mutex_lock(&spidev->buf_lock);
 	status = spidev_sync_read(spidev, count);
-	if (status == 0) {
+	if (status > 0) {
 		unsigned long	missing;
 
-		missing = copy_to_user(buf, spidev->buffer, count);
-		if (count && missing == count)
+		missing = copy_to_user(buf, spidev->buffer, status);
+		if (missing == status)
 			status = -EFAULT;
 		else
-			status = count - missing;
+			status = status - missing;
 	}
 	mutex_unlock(&spidev->buf_lock);
 
@@ -200,8 +200,6 @@ spidev_write(struct file *filp, const char __user *buf,
 	missing = copy_from_user(spidev->buffer, buf, count);
 	if (missing == 0) {
 		status = spidev_sync_write(spidev, count);
-		if (status == 0)
-			status = count;
 	} else
 		status = -EFAULT;
 	mutex_unlock(&spidev->buf_lock);