block: don't allow enabling a cache on devices that don't support it

Currently the write_cache attribute allows enabling the QUEUE_FLAG_WC
flag on devices that never claimed the capability.

Fix that by adding a QUEUE_FLAG_HW_WC flag that is set by
blk_queue_write_cache and guards re-enabling the cache through sysfs.

Note that any rescan that calls blk_queue_write_cache will still
re-enable the write cache as in the current code.

Fixes: 93e9d8e ("block: add ability to flag write back caching on a device")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230707094239.107968-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-settings.c

@@ -830,10 +830,13 @@ EXPORT_SYMBOL(blk_set_queue_depth);
  */
 void blk_queue_write_cache(struct request_queue *q, bool wc, bool fua)
 {
-	if (wc)
+	if (wc) {
+		blk_queue_flag_set(QUEUE_FLAG_HW_WC, q);
 		blk_queue_flag_set(QUEUE_FLAG_WC, q);
-	else
+	} else {
+		blk_queue_flag_clear(QUEUE_FLAG_HW_WC, q);
 		blk_queue_flag_clear(QUEUE_FLAG_WC, q);
+	}
 	if (fua)
 		blk_queue_flag_set(QUEUE_FLAG_FUA, q);
 	else

block/blk-sysfs.c

@@ -449,13 +449,16 @@ static ssize_t queue_wc_show(struct request_queue *q, char *page)
 static ssize_t queue_wc_store(struct request_queue *q, const char *page,
 			      size_t count)
 {
-	if (!strncmp(page, "write back", 10))
+	if (!strncmp(page, "write back", 10)) {
+		if (!test_bit(QUEUE_FLAG_HW_WC, &q->queue_flags))
+			return -EINVAL;
 		blk_queue_flag_set(QUEUE_FLAG_WC, q);
-	else if (!strncmp(page, "write through", 13) ||
-		 !strncmp(page, "none", 4))
+	} else if (!strncmp(page, "write through", 13) ||
+		 !strncmp(page, "none", 4)) {
 		blk_queue_flag_clear(QUEUE_FLAG_WC, q);
-	else
+	} else {
 		return -EINVAL;
+	}
 
 	return count;
 }

include/linux/blkdev.h

@@ -538,6 +538,7 @@ struct request_queue {
 #define QUEUE_FLAG_ADD_RANDOM	10	/* Contributes to random pool */
 #define QUEUE_FLAG_SYNCHRONOUS	11	/* always completes in submit context */
 #define QUEUE_FLAG_SAME_FORCE	12	/* force complete on same CPU */
+#define QUEUE_FLAG_HW_WC	18	/* Write back caching supported */
 #define QUEUE_FLAG_INIT_DONE	14	/* queue is initialized */
 #define QUEUE_FLAG_STABLE_WRITES 15	/* don't modify blks until WB is done */
 #define QUEUE_FLAG_POLL		16	/* IO polling enabled if set */