mmc: alcor: fix DMA reads

Setting max_blk_count to 1 here was causing the mmc block layer
to always use the MMC_READ_SINGLE_BLOCK command here, which the
driver does not DMA-accelerate.

Drop the max_blk_ settings here. The mmc host defaults suffice,
along with the max_segs and max_seg_size settings, which I have
now documented in more detail.

Now each MMC command reads 4 512-byte blocks, using DMA instead of
PIO. On my SD card, this increases read performance (measured with dd)
from 167kb/sec to 4.6mb/sec.

Link: http://lkml.kernel.org/r/CAD8Lp47L5T3jnAjBiPs1cQ+yFA3L6LJtgFvMETnBrY63-Zdi2g@mail.gmail.com
Signed-off-by: Daniel Drake <[email protected]>
Reviewed-by: Oleksij Rempel <[email protected]>
Fixes: c5413ad ("mmc: add new Alcor Micro Cardreader SD/MMC driver")
Cc: [email protected]
Signed-off-by: Ulf Hansson <[email protected]>

drivers/mmc/host/alcor.c

@@ -1044,14 +1044,27 @@ static void alcor_init_mmc(struct alcor_sdmmc_host *host)
 	mmc->caps2 = MMC_CAP2_NO_SDIO;
 	mmc->ops = &alcor_sdc_ops;
 
-	/* Hardware cannot do scatter lists */
+	/* 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.
+	 *
+	 * 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.
+	 */
 	mmc->max_segs = AU6601_MAX_DMA_SEGMENTS;
 	mmc->max_seg_size = AU6601_MAX_DMA_BLOCK_SIZE;
-
-	mmc->max_blk_size = mmc->max_seg_size;
-	mmc->max_blk_count = mmc->max_segs;
-
-	mmc->max_req_size = mmc->max_seg_size * mmc->max_segs;
+	mmc->max_req_size = mmc->max_seg_size;
 }
 
 static int alcor_pci_sdmmc_drv_probe(struct platform_device *pdev)