[[RFC] 1/5] MMC: Adjust unaligned write accesses.

[Andrei Warkentin <andreiw@motorola.com>]

Adjust unaligned write accesses spanning preferred align
size into two accesses - an unaligned and an aligned access.
This is meant to be used for card quirks, and is off
by default. A limiting value in transfer size
for this adjustment is available, as on some cards there is a
perf decrease for larger transfers.

Signed-off-by: Andrei Warkentin <andreiw@motorola.com>
---
 drivers/mmc/card/block.c |   43 +++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 43 insertions(+), 0 deletions(-)

diff --git a/drivers/mmc/card/block.c b/drivers/mmc/card/block.c
index 7054fd5..9d44480 100644
--- a/drivers/mmc/card/block.c
+++ b/drivers/mmc/card/block.c
@@ -63,6 +63,8 @@ struct mmc_blk_data {
 
 	unsigned int	usage;
 	unsigned int	read_only;
+	unsigned int	write_align_size;
+	unsigned int	write_align_limit;
 };
 
 static DEFINE_MUTEX(open_lock);
@@ -312,6 +314,43 @@ out:
 	return err ? 0 : 1;
 }
 
+/*
+ * If the request is not aligned, split it into an unaligned
+ * and an aligned portion. Here we can adjust
+ * the size of the MMC request and let the block layer request handle
+ * deal with generating another MMC request.
+ */
+
+static void mmc_adjust_write(struct mmc_card *card,
+			     struct mmc_request *mrq)
+{
+	unsigned int left_in_page;
+	unsigned int wa_size_blocks;
+	struct mmc_blk_data *md = mmc_get_drvdata(card);
+
+	if (!md->write_align_size)
+		return;
+
+	if (md->write_align_limit &&
+	    (md->write_align_limit / mrq->data->blksz)
+	    < mrq->data->blocks)
+		return;
+
+	wa_size_blocks = md->write_align_size / mrq->data->blksz;
+	left_in_page = wa_size_blocks -
+		(mrq->cmd->arg % wa_size_blocks);
+
+	/* Aligned access. */
+	if (left_in_page == wa_size_blocks)
+		return;
+
+	/* Not straddling page boundary. */
+	if (mrq->data->blocks <= left_in_page)
+		return;
+
+	mrq->data->blocks = left_in_page;
+}
+
 static int mmc_blk_issue_rw_rq(struct mmc_queue *mq, struct request *req)
 {
 	struct mmc_blk_data *md = mq->data;
@@ -339,6 +378,10 @@ static int mmc_blk_issue_rw_rq(struct mmc_queue *mq, struct request *req)
 		brq.stop.flags = MMC_RSP_SPI_R1B | MMC_RSP_R1B | MMC_CMD_AC;
 		brq.data.blocks = blk_rq_sectors(req);
 
+		/* Check for unaligned accesses straddling pages. */
+		if (rq_data_dir(req) == WRITE)
+			mmc_adjust_write(card, &brq.mrq);
+
 		/*
 		 * The block layer doesn't support all sector count
 		 * restrictions, so we need to be prepared for too big
-- 
1.7.0.4