[PATCH v2 1/2] mmc: dw_mmc: implement option for configuring DMA threshold

[Kaustabh Chakraborty]

Some controllers, such as certain Exynos SDIO ones, are unable to
perform DMA transfers of small amount of bytes properly. Following the
device tree schema, implement the property to define the DMA transfer
threshold (from a hard coded value of 16 bytes) so that lesser number of
bytes can be transferred safely skipping DMA in such controllers. The
value of 16 bytes stays as the default for controllers which do not
define it. This value can be overridden by implementation-specific init
sequences.

Signed-off-by: Kaustabh Chakraborty <kauschluss@xxxxxxxxxxx>
---
 drivers/mmc/host/dw_mmc.c | 5 +++--
 drivers/mmc/host/dw_mmc.h | 2 ++
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
index 20193ee7b73eb..9dd9fed4ccf49 100644
--- a/drivers/mmc/host/dw_mmc.c
+++ b/drivers/mmc/host/dw_mmc.c
@@ -40,7 +40,6 @@
 				 SDMMC_INT_RESP_ERR | SDMMC_INT_HLE)
 #define DW_MCI_ERROR_FLAGS	(DW_MCI_DATA_ERROR_FLAGS | \
 				 DW_MCI_CMD_ERROR_FLAGS)
-#define DW_MCI_DMA_THRESHOLD	16
 
 #define DW_MCI_FREQ_MAX	200000000	/* unit: HZ */
 #define DW_MCI_FREQ_MIN	100000		/* unit: HZ */
@@ -821,7 +820,7 @@ static int dw_mci_pre_dma_transfer(struct dw_mci *host,
 	 * non-word-aligned buffers or lengths. Also, we don't bother
 	 * with all the DMA setup overhead for short transfers.
 	 */
-	if (data->blocks * data->blksz < DW_MCI_DMA_THRESHOLD)
+	if (data->blocks * data->blksz < host->dma_threshold)
 		return -EINVAL;
 
 	if (data->blksz & 3)
@@ -3245,6 +3244,8 @@ int dw_mci_probe(struct dw_mci *host)
 		goto err_clk_ciu;
 	}
 
+	host->dma_threshold = 16;
+
 	if (host->rstc) {
 		reset_control_assert(host->rstc);
 		usleep_range(10, 50);
diff --git a/drivers/mmc/host/dw_mmc.h b/drivers/mmc/host/dw_mmc.h
index 42e58be74ce09..fc7601fba849f 100644
--- a/drivers/mmc/host/dw_mmc.h
+++ b/drivers/mmc/host/dw_mmc.h
@@ -164,6 +164,8 @@ struct dw_mci {
 	void __iomem		*fifo_reg;
 	u32			data_addr_override;
 	bool			wm_aligned;
+	/* Configurable data byte threshold value for DMA transfer. */
+	u32			dma_threshold;
 
 	struct scatterlist	*sg;
 	struct sg_mapping_iter	sg_miter;

-- 
2.53.0