mmc: remove bondage between REQ_META and reliable write

Anytime a write operation is performed with Reliable Write flag enabled,
the eMMC device is enforced to bypass the cache and do a write to the
underling NVM device by Jedec specification; this causes a performance
penalty since write operations can't be optimized by the device cache.

In our tests, we replayed a typical mobile daily trace pattern and found
~9% overall time reduction in trace replay by using this patch. Also the
write ops within 4KB~64KB chunk size range get a 40~60% performance
improvement by using the patch (as this range of write chunks are the ones
affected by REQ_META).

This patch has been discussed in the Mobile & Embedded Linux Storage Forum
and it's the results of feedbacks from many people. We also checked with
fsdevl and f2fs mailing list developers that this change in the usage of
REQ_META is not affecting FS behavior and we got positive feedbacks.
Reporting here the feedbacks:
http://comments.gmane.org/gmane.linux.file-systems/97219
http://thread.gmane.org/gmane.linux.file-systems.f2fs/3178/focus=3183

Signed-off-by: Bruce Ford <bford@micron.com>
Signed-off-by: Luca Porzio <lporzio@micron.com>
Fixes: ce39f9d17c14 ("mmc: support packed write command for eMMC4.5 devices")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/card/block.c

@@ -65,8 +65,7 @@ MODULE_ALIAS("mmc:block");
 #define MMC_SANITIZE_REQ_TIMEOUT 240000
 #define MMC_EXTRACT_INDEX_FROM_ARG(x) ((x & 0x00FF0000) >> 16)
 
-#define mmc_req_rel_wr(req)	(((req->cmd_flags & REQ_FUA) || \
-				  (req->cmd_flags & REQ_META)) && \
+#define mmc_req_rel_wr(req)	((req->cmd_flags & REQ_FUA) && \
 				  (rq_data_dir(req) == WRITE))
 #define PACKED_CMD_VER	0x01
 #define PACKED_CMD_WR	0x02
@@ -1467,13 +1466,9 @@ static void mmc_blk_rw_rq_prep(struct mmc_queue_req *mqrq,
 
 	/*
 	 * Reliable writes are used to implement Forced Unit Access and
-	 * REQ_META accesses, and are supported only on MMCs.
-	 *
-	 * XXX: this really needs a good explanation of why REQ_META
-	 * is treated special.
+	 * are supported only on MMCs.
 	 */
-	bool do_rel_wr = ((req->cmd_flags & REQ_FUA) ||
-			  (req->cmd_flags & REQ_META)) &&
+	bool do_rel_wr = (req->cmd_flags & REQ_FUA) &&
 		(rq_data_dir(req) == WRITE) &&
 		(md->flags & MMC_BLK_REL_WR);