+static u8 *add_bytestring_header(int *err, struct opal_dev *cmd, size_t len)
 {
 	size_t header_len = 1;
 	bool is_short_atom = true;
-
-	if (*err)
-		return;
+	char *start;

Shouldn't this also return early if we have a pending error?

Except for that this looks fine:

Reviewed-by: Christoph Hellwig <hch@lst.de>