[PATCH 2/2] mmc: mmci: stm32: use a buffer for unaligned DMA requests

[Yann Gautier]

In SDIO mode, the sg list for requests can be unaligned with what the
STM32 SDMMC internal DMA can support. In that case, instead of failing,
use a temporary bounce buffer to copy from/to the sg list.
This buffer is limited to 1MB. But for that we need to also limit
max_req_size to 1MB. It has not shown any throughput penalties for
SD-cards or eMMC.

Signed-off-by: Yann Gautier <yann.gautier@xxxxxxxxxxx>
---
 drivers/mmc/host/mmci_stm32_sdmmc.c | 80 +++++++++++++++++++++++------
 1 file changed, 63 insertions(+), 17 deletions(-)

diff --git a/drivers/mmc/host/mmci_stm32_sdmmc.c b/drivers/mmc/host/mmci_stm32_sdmmc.c
index 4566d7fc9055..a4414e32800f 100644
--- a/drivers/mmc/host/mmci_stm32_sdmmc.c
+++ b/drivers/mmc/host/mmci_stm32_sdmmc.c
@@ -43,6 +43,9 @@ struct sdmmc_lli_desc {
 struct sdmmc_idma {
 	dma_addr_t sg_dma;
 	void *sg_cpu;
+	dma_addr_t bounce_dma_addr;
+	void *bounce_buf;
+	bool use_bounce_buffer;
 };
 
 struct sdmmc_dlyb {
@@ -54,6 +57,7 @@ struct sdmmc_dlyb {
 static int sdmmc_idma_validate_data(struct mmci_host *host,
 				    struct mmc_data *data)
 {
+	struct sdmmc_idma *idma = host->dma_priv;
 	struct scatterlist *sg;
 	int i;
 
@@ -61,21 +65,23 @@ static int sdmmc_idma_validate_data(struct mmci_host *host,
 	 * idma has constraints on idmabase & idmasize for each element
 	 * excepted the last element which has no constraint on idmasize
 	 */
+	idma->use_bounce_buffer = false;
 	for_each_sg(data->sg, sg, data->sg_len - 1, i) {
 		if (!IS_ALIGNED(sg->offset, sizeof(u32)) ||
 		    !IS_ALIGNED(sg->length, SDMMC_IDMA_BURST)) {
-			dev_err(mmc_dev(host->mmc),
+			dev_dbg(mmc_dev(host->mmc),
 				"unaligned scatterlist: ofst:%x length:%d\n",
 				data->sg->offset, data->sg->length);
-			return -EINVAL;
+			idma->use_bounce_buffer = true;
+			return 0;
 		}
 	}
 
 	if (!IS_ALIGNED(sg->offset, sizeof(u32))) {
-		dev_err(mmc_dev(host->mmc),
+		dev_dbg(mmc_dev(host->mmc),
 			"unaligned last scatterlist: ofst:%x length:%d\n",
 			data->sg->offset, data->sg->length);
-		return -EINVAL;
+		idma->use_bounce_buffer = true;
 	}
 
 	return 0;
@@ -84,18 +90,29 @@ static int sdmmc_idma_validate_data(struct mmci_host *host,
 static int _sdmmc_idma_prep_data(struct mmci_host *host,
 				 struct mmc_data *data)
 {
-	int n_elem;
+	struct sdmmc_idma *idma = host->dma_priv;
 
-	n_elem = dma_map_sg(mmc_dev(host->mmc),
-			    data->sg,
-			    data->sg_len,
-			    mmc_get_dma_dir(data));
+	if (idma->use_bounce_buffer) {
+		if (data->flags & MMC_DATA_WRITE) {
+			unsigned int xfer_bytes = data->blksz * data->blocks;
 
-	if (!n_elem) {
-		dev_err(mmc_dev(host->mmc), "dma_map_sg failed\n");
-		return -EINVAL;
-	}
+			sg_copy_to_buffer(data->sg, data->sg_len,
+					  idma->bounce_buf, xfer_bytes);
+			dma_wmb();
+		}
+	} else {
+		int n_elem;
+
+		n_elem = dma_map_sg(mmc_dev(host->mmc),
+				    data->sg,
+				    data->sg_len,
+				    mmc_get_dma_dir(data));
 
+		if (!n_elem) {
+			dev_err(mmc_dev(host->mmc), "dma_map_sg failed\n");
+			return -EINVAL;
+		}
+	}
 	return 0;
 }
 
@@ -112,8 +129,19 @@ static int sdmmc_idma_prep_data(struct mmci_host *host,
 static void sdmmc_idma_unprep_data(struct mmci_host *host,
 				   struct mmc_data *data, int err)
 {
-	dma_unmap_sg(mmc_dev(host->mmc), data->sg, data->sg_len,
-		     mmc_get_dma_dir(data));
+	struct sdmmc_idma *idma = host->dma_priv;
+
+	if (idma->use_bounce_buffer) {
+		if (data->flags & MMC_DATA_READ) {
+			unsigned int xfer_bytes = data->blksz * data->blocks;
+
+			sg_copy_from_buffer(data->sg, data->sg_len,
+					    idma->bounce_buf, xfer_bytes);
+		}
+	} else {
+		dma_unmap_sg(mmc_dev(host->mmc), data->sg, data->sg_len,
+			     mmc_get_dma_dir(data));
+	}
 }
 
 static int sdmmc_idma_setup(struct mmci_host *host)
@@ -137,6 +165,16 @@ static int sdmmc_idma_setup(struct mmci_host *host)
 		host->mmc->max_segs = SDMMC_LLI_BUF_LEN /
 			sizeof(struct sdmmc_lli_desc);
 		host->mmc->max_seg_size = host->variant->stm32_idmabsize_mask;
+
+		host->mmc->max_req_size = SZ_1M;
+		idma->bounce_buf = dmam_alloc_coherent(dev,
+						       host->mmc->max_req_size,
+						       &idma->bounce_dma_addr,
+						       GFP_KERNEL);
+		if (!idma->bounce_buf) {
+			dev_err(dev, "Unable to map allocate DMA bounce buffer.\n");
+			return -ENOMEM;
+		}
 	} else {
 		host->mmc->max_segs = 1;
 		host->mmc->max_seg_size = host->mmc->max_req_size;
@@ -154,8 +192,16 @@ static int sdmmc_idma_start(struct mmci_host *host, unsigned int *datactrl)
 	struct scatterlist *sg;
 	int i;
 
-	if (!host->variant->dma_lli || data->sg_len == 1) {
-		writel_relaxed(sg_dma_address(data->sg),
+	if (!host->variant->dma_lli || data->sg_len == 1 ||
+	    idma->use_bounce_buffer) {
+		u32 dma_addr;
+
+		if (idma->use_bounce_buffer)
+			dma_addr = idma->bounce_dma_addr;
+		else
+			dma_addr = sg_dma_address(data->sg);
+
+		writel_relaxed(dma_addr,
 			       host->base + MMCI_STM32_IDMABASE0R);
 		writel_relaxed(MMCI_STM32_IDMAEN,
 			       host->base + MMCI_STM32_IDMACTRLR);
-- 
2.25.1