[PATCH 7/8] spi: mtk_snor: fix zeroed data in DMA read bounce path

[David Lechner]

From: Macpaul Lin <macpaul.lin at mediatek.com>

Implement proper bounce buffer handling for the read path to fix zeroed
data when using DMA. In the bounce path, map the bounce buffer with
dma_map_single(), perform DMA using bounce_dma, then copy data from the
bounce buffer to the user buffer, and finally unmap with
dma_unmap_single().

Signed-off-by: Macpaul Lin <macpaul.lin at mediatek.com>
Signed-off-by: David Lechner <dlechner at baylibre.com>
---
 drivers/spi/mtk_snor.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/spi/mtk_snor.c b/drivers/spi/mtk_snor.c
index 649bca5716c..21ac115e3c3 100644
--- a/drivers/spi/mtk_snor.c
+++ b/drivers/spi/mtk_snor.c
@@ -274,6 +274,7 @@ static int mtk_snor_read_bounce(struct mtk_snor_priv *priv,
 {
 	unsigned int rdlen;
 	int ret;
+	dma_addr_t bounce_dma;
 
 	if (op->data.nbytes & MTK_NOR_DMA_ALIGN_MASK)
 		rdlen = (op->data.nbytes + MTK_NOR_DMA_ALIGN) &
@@ -281,11 +282,21 @@ static int mtk_snor_read_bounce(struct mtk_snor_priv *priv,
 	else
 		rdlen = op->data.nbytes;
 
-	ret = mtk_snor_dma_exec(priv, op->addr.val, rdlen,
-				(dma_addr_t)priv->buffer);
+	/* Map bounce buffer for DMA */
+	bounce_dma = dma_map_single(priv->buffer, rdlen, DMA_FROM_DEVICE);
+	if (dma_mapping_error(priv->dev, bounce_dma)) {
+		dev_err(priv->dev, "bounce buffer dma map failed\n");
+		return -EINVAL;
+	}
 
-	if (!ret)
+	ret = mtk_snor_dma_exec(priv, op->addr.val, rdlen, bounce_dma);
+	/* Ensure DMA writes are visible to CPU and copy the requested bytes */
+	if (!ret) {
+		/* Synchronize cached data to CPU visible memory if needed */
 		memcpy(op->data.buf.in, priv->buffer, op->data.nbytes);
+	}
+	/* Unmap bounce buffer regardless of success/failure */
+	dma_unmap_single(bounce_dma, rdlen, DMA_FROM_DEVICE);
 
 	return ret;
 }

-- 
2.43.0