[RFC PATCH 1/2] mtd: spi-nor: Introduce bounce buffer to handle vmalloc'd buffers

[Vignesh R]

Filesystems like UBIFS may pass vmalloc'd buffers to SPI NOR layer which
will end up in SPI layer. SPI core does try to handle such buffers (see
spi_map_buf()) by doing vmalloc_to_page() and creating scatterlist. But,
its known that this does not work well with VIVT/aliasing cache
architectures.
This also fails when buffers are addressed using LPAE (buffers in region
higher than 32 bit addressable region), if DMA is 32bit only.

Introduce bounce buffers support in SPI NOR framework to handle
vmalloc'd buffers. Use a pre-allocated per flash bounce buffer equal to
the sector size of the flash. Flash drivers can enable this feature by
setting SNOR_F_USE_BOUNCE_BUFFER flag.
This would also enable SPI NOR drivers to safely use DMA in their
read/write callbacks.

Signed-off-by: Vignesh R <vigneshr@xxxxxx>
---
 drivers/mtd/spi-nor/spi-nor.c | 30 +++++++++++++++++++++++++++---
 include/linux/mtd/spi-nor.h   |  4 ++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
index 747645c74134..c241fefa5aff 100644
--- a/drivers/mtd/spi-nor/spi-nor.c
+++ b/drivers/mtd/spi-nor/spi-nor.c
@@ -17,6 +17,7 @@
 #include <linux/mutex.h>
 #include <linux/math64.h>
 #include <linux/sizes.h>
+#include <linux/mm.h>
 
 #include <linux/mtd/mtd.h>
 #include <linux/of_platform.h>
@@ -1205,11 +1206,21 @@ static int spi_nor_read(struct mtd_info *mtd, loff_t from, size_t len,
 
 	while (len) {
 		loff_t addr = from;
+		bool use_bb = false;
+		u_char *dst_buf = buf;
+		size_t buf_len = len;
 
 		if (nor->flags & SNOR_F_S3AN_ADDR_DEFAULT)
 			addr = spi_nor_s3an_addr_convert(nor, addr);
 
-		ret = nor->read(nor, addr, len, buf);
+		if (!virt_addr_valid(buf) && nor->bounce_buf) {
+			use_bb = true;
+			dst_buf = nor->bounce_buf;
+			if (len > mtd->erasesize)
+				buf_len = mtd->erasesize;
+		}
+
+		ret = nor->read(nor, from, buf_len, dst_buf);
 		if (ret == 0) {
 			/* We shouldn't see 0-length reads */
 			ret = -EIO;
@@ -1217,7 +1228,8 @@ static int spi_nor_read(struct mtd_info *mtd, loff_t from, size_t len,
 		}
 		if (ret < 0)
 			goto read_err;
-
+		if (use_bb)
+			memcpy(buf, dst_buf, ret);
 		WARN_ON(ret > len);
 		*retlen += ret;
 		buf += ret;
@@ -1329,6 +1341,7 @@ static int spi_nor_write(struct mtd_info *mtd, loff_t to, size_t len,
 		return ret;
 
 	for (i = 0; i < len; ) {
+		const u_char *src_buf = buf + i;
 		ssize_t written;
 		loff_t addr = to + i;
 
@@ -1354,8 +1367,13 @@ static int spi_nor_write(struct mtd_info *mtd, loff_t to, size_t len,
 		if (nor->flags & SNOR_F_S3AN_ADDR_DEFAULT)
 			addr = spi_nor_s3an_addr_convert(nor, addr);
 
+		if (!virt_addr_valid(buf) && nor->bounce_buf) {
+			memcpy(nor->bounce_buf, buf + i, page_remain);
+			src_buf = nor->bounce_buf;
+		}
+
 		write_enable(nor);
-		ret = nor->write(nor, addr, page_remain, buf + i);
+		ret = nor->write(nor, addr, page_remain, src_buf);
 		if (ret < 0)
 			goto write_err;
 		written = ret;
@@ -1720,6 +1738,12 @@ int spi_nor_scan(struct spi_nor *nor, const char *name, enum read_mode mode)
 		return -EINVAL;
 	}
 
+	if (nor->flags & SNOR_F_USE_BOUNCE_BUFFER) {
+		nor->bounce_buf = devm_kmalloc(dev, mtd->erasesize, GFP_KERNEL);
+		if (!nor->bounce_buf)
+			dev_err(dev, "unable to allocated bounce buffer\n");
+	}
+
 	nor->read_dummy = spi_nor_read_dummy_cycles(nor);
 
 	if (info->flags & SPI_S3AN) {
diff --git a/include/linux/mtd/spi-nor.h b/include/linux/mtd/spi-nor.h
index f2a718030476..3846ff5fa011 100644
--- a/include/linux/mtd/spi-nor.h
+++ b/include/linux/mtd/spi-nor.h
@@ -141,6 +141,7 @@ enum spi_nor_option_flags {
 	SNOR_F_NO_OP_CHIP_ERASE	= BIT(2),
 	SNOR_F_S3AN_ADDR_DEFAULT = BIT(3),
 	SNOR_F_READY_XSR_RDY	= BIT(4),
+	SNOR_F_USE_BOUNCE_BUFFER = BIT(5),
 };
 
 /**
@@ -173,6 +174,8 @@ enum spi_nor_option_flags {
  * @flash_unlock:	[FLASH-SPECIFIC] unlock a region of the SPI NOR
  * @flash_is_locked:	[FLASH-SPECIFIC] check if a region of the SPI NOR is
  *			completely locked
+ * @bounce_buf		bounce buffer to use incase of vmalloc'd buffers,
+ *			for drivers that may use DMA.
  * @priv:		the private data
  */
 struct spi_nor {
@@ -205,6 +208,7 @@ struct spi_nor {
 	int (*flash_unlock)(struct spi_nor *nor, loff_t ofs, uint64_t len);
 	int (*flash_is_locked)(struct spi_nor *nor, loff_t ofs, uint64_t len);
 
+	void *bounce_buf;
 	void *priv;
 };
 
-- 
2.11.0