[PATCH] block: don't error while flushing when write support is disabled

["Enrico Jörns" <ejo@pengutronix.de>]

If write support for the underlying device is disabled (e.g. by
disabling CONFIG_MCI_WRITE), blk->ops->write will be NULL.
Unconditionally dereferencing/calling blk->ops->write() results in a
NULL pointer dereference.

Fix this by making chunk_flush() a noop for missing write support.
Don't return an error since there should be nothing to flush if write
support is disabled, anyway.

Note that barebox currently does not prevent calling write/flush
operations, even if CONFIG_MCI_WRITE and thus write support is actually
disabled.
But this should be handled on another level.

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>
---
 common/block.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/common/block.c b/common/block.c
index ca2ed37dbd..abd05eab39 100644
--- a/common/block.c
+++ b/common/block.c
@@ -60,6 +60,9 @@ static int chunk_flush(struct block_device *blk, struct chunk *chunk)
 	if (!chunk->dirty)
 		return 0;
 
+	if (!blk->ops->write)
+		return 0;
+
 	len = writebuffer_io_len(blk, chunk);
 	ret = blk->ops->write(blk, chunk->data, chunk->block_start, len);
 	if (ret < 0)
-- 
2.47.3