mmc: alcor: work with multiple-entry sglists

DMA on this hardware is limited to dealing with a 4096 bytes at a
time. Previously, the driver was set up accordingly to request single-page
DMA buffers, however that had the effect of generating a large number
of small MMC requests for data I/O.

Improve the driver to accept multi-entry scatter-gather lists. The size of
each entry is already capped to 4096 bytes (AU6601_MAX_DMA_BLOCK_SIZE),
matching the hardware requirements. Existing driver code already iterates
through remaining sglist entries after each DMA transfer is complete.

Also add some comments to help clarify the situation, and clear up
some of the confusion I had regarding DMA vs PIO.

Testing with dd, this increases write performance from 2mb/sec to
10mb/sec, and increases read performance from 4mb/sec to 14mb/sec.

Signed-off-by: Daniel Drake <drake@endlessm.com>
Link: http://lkml.kernel.org/r/CAD8Lp47JYdZzbV9F+asNwvSfLF_po_J7ir6R_Vb-Dab21_=Krw@mail.gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

1 files changed, 36 insertions, 18 deletions

diff --git a/drivers/mmc/host/alcor.c b/drivers/mmc/host/alcor.c
index 3e2efbf3cf8c..4b2690756453 100644
--- a/drivers/mmc/host/alcor.c
+++ b/drivers/mmc/host/alcor.c
@@ -117,6 +117,9 @@ static void alcor_reset(struct alcor_sdmmc_host *host, u8 val)
 	dev_err(host->dev, "%s: timeout\n", __func__);
 }
 
+/*
+ * Perform DMA I/O of a single page.
+ */
 static void alcor_data_set_dma(struct alcor_sdmmc_host *host)
 {
 	struct alcor_pci_priv *priv = host->alcor_pci;
@@ -153,12 +156,26 @@ static void alcor_trigger_data_transfer(struct alcor_sdmmc_host *host)
 		ctrl |= AU6601_DATA_WRITE;
 
 	if (data->host_cookie == COOKIE_MAPPED) {
+		/*
+		 * For DMA transfers, this function is called just once,
+		 * at the start of the operation. The hardware can only
+		 * perform DMA I/O on a single page at a time, so here
+		 * we kick off the transfer with the first page, and expect
+		 * subsequent pages to be transferred upon IRQ events
+		 * indicating that the single-page DMA was completed.
+		 */
 		alcor_data_set_dma(host);
 		ctrl |= AU6601_DATA_DMA_MODE;
 		host->dma_on = 1;
 		alcor_write32(priv, data->sg_count * 0x1000,
 			       AU6601_REG_BLOCK_SIZE);
 	} else {
+		/*
+		 * For PIO transfers, we break down each operation
+		 * into several sector-sized transfers. When one sector has
+		 * complete, the IRQ handler will call this function again
+		 * to kick off the transfer of the next sector.
+		 */
 		alcor_write32(priv, data->blksz, AU6601_REG_BLOCK_SIZE);
 	}
 
@@ -776,8 +793,12 @@ static void alcor_pre_req(struct mmc_host *mmc,
 		return;
 	/*
 	 * We don't do DMA on "complex" transfers, i.e. with
-	 * non-word-aligned buffers or lengths. Also, we don't bother
-	 * with all the DMA setup overhead for short transfers.
+	 * non-word-aligned buffers or lengths. A future improvement
+	 * could be made to use temporary DMA bounce-buffers when these
+	 * requirements are not met.
+	 *
+	 * Also, we don't bother with all the DMA setup overhead for
+	 * short transfers.
 	 */
 	if (data->blocks * data->blksz < AU6601_MAX_DMA_BLOCK_SIZE)
 		return;
@@ -788,6 +809,8 @@ static void alcor_pre_req(struct mmc_host *mmc,
 	for_each_sg(data->sg, sg, data->sg_len, i) {
 		if (sg->length != AU6601_MAX_DMA_BLOCK_SIZE)
 			return;
+		if (sg->offset != 0)
+			return;
 	}
 
 	/* This data might be unmapped at this time */
@@ -1038,26 +1061,21 @@ static void alcor_init_mmc(struct alcor_sdmmc_host *host)
 	mmc->ops = &alcor_sdc_ops;
 
 	/* The hardware does DMA data transfer of 4096 bytes to/from a single
-	 * buffer address. Scatterlists are not supported, but upon DMA
-	 * completion (signalled via IRQ), the original vendor driver does
-	 * then immediately set up another DMA transfer of the next 4096
-	 * bytes.
-	 *
-	 * This means that we need to handle the I/O in 4096 byte chunks.
-	 * Lacking a way to limit the sglist entries to 4096 bytes, we instead
-	 * impose that only one segment is provided, with maximum size 4096,
-	 * which also happens to be the minimum size. This means that the
-	 * single-entry sglist handled by this driver can be handed directly
-	 * to the hardware, nice and simple.
+	 * buffer address. Scatterlists are not supported at the hardware
+	 * level, however we can work with them at the driver level,
+	 * provided that each segment is exactly 4096 bytes in size.
+	 * Upon DMA completion of a single segment (signalled via IRQ), we
+	 * immediately proceed to transfer the next segment from the
+	 * scatterlist.
 	 *
-	 * Unfortunately though, that means we only do 4096 bytes I/O per
-	 * MMC command. A future improvement would be to make the driver
-	 * accept sg lists and entries of any size, and simply iterate
-	 * through them 4096 bytes at a time.
+	 * The overall request is limited to 240 sectors, matching the
+	 * original vendor driver.
 	 */
 	mmc->max_segs = AU6601_MAX_DMA_SEGMENTS;
 	mmc->max_seg_size = AU6601_MAX_DMA_BLOCK_SIZE;
-	mmc->max_req_size = mmc->max_seg_size;
+	mmc->max_blk_count = 240;
+	mmc->max_req_size = mmc->max_blk_count * mmc->max_blk_size;
+	dma_set_max_seg_size(host->dev, mmc->max_seg_size);
 }
 
 static int alcor_pci_sdmmc_drv_probe(struct platform_device *pdev)