[PATCH v2 6/6] mtd: spi-nor: core: avoid odd length/address writes in 8D-8D-8D mode

[Pratyush Yadav <p.yadav@xxxxxx>]

On Octal DTR capable flashes like Micron Xcella the writes cannot start
or end at an odd address in Octal DTR mode. Extra 0xff bytes need to be
appended or prepended to make sure the start address and end address are
even. 0xff is used because on NOR flashes a program operation can only
flip bits from 1 to 0, not the other way round. 0 to 1 flip needs to
happen via erases.

Signed-off-by: Pratyush Yadav <p.yadav@xxxxxx>

---

Changes in v2:
- Replace semicolon in subject with colon.
- Add a comment that ret < 0 after adjusting for extra bytes is not
  possible, and add a WARN_ON() on the condition to make sure it gets
  spotted quickly when some change triggers this bug.

 drivers/mtd/spi-nor/core.c | 73 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 72 insertions(+), 1 deletion(-)

diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
index a696af6a1b71..d2a7e16e667d 100644
--- a/drivers/mtd/spi-nor/core.c
+++ b/drivers/mtd/spi-nor/core.c
@@ -2023,6 +2023,72 @@ static int spi_nor_read(struct mtd_info *mtd, loff_t from, size_t len,
 	return ret;
 }
 
+/*
+ * On Octal DTR capable flashes like Micron Xcella the writes cannot start or
+ * end at an odd address in Octal DTR mode. Extra 0xff bytes need to be appended
+ * or prepended to make sure the start address and end address are even. 0xff is
+ * used because on NOR flashes a program operation can only flip bits from 1 to
+ * 0, not the other way round. 0 to 1 flip needs to happen via erases.
+ */
+static int spi_nor_octal_dtr_write(struct spi_nor *nor, loff_t to, size_t len,
+				   const u8 *buf)
+{
+	u8 *tmp_buf;
+	size_t bytes_written;
+	loff_t start, end;
+	int ret;
+
+	if (IS_ALIGNED(to, 2) && IS_ALIGNED(len, 2))
+		return spi_nor_write_data(nor, to, len, buf);
+
+	tmp_buf = kmalloc(nor->page_size, GFP_KERNEL);
+	if (!tmp_buf)
+		return -ENOMEM;
+
+	memset(tmp_buf, 0xff, nor->page_size);
+
+	start = round_down(to, 2);
+	end = round_up(to + len, 2);
+
+	memcpy(tmp_buf + (to - start), buf, len);
+
+	ret = spi_nor_write_data(nor, start, end - start, tmp_buf);
+	if (ret == 0) {
+		ret = -EIO;
+		goto out;
+	}
+	if (ret < 0)
+		goto out;
+
+	/*
+	 * More bytes are written than actually requested, but that number can't
+	 * be reported to the calling function or it will confuse its
+	 * calculations. Calculate how many of the _requested_ bytes were
+	 * written.
+	 */
+	bytes_written = ret;
+
+	if (to != start)
+		ret -= to - start;
+
+	/*
+	 * Only account for extra bytes at the end if they were actually
+	 * written. For example, if for some reason the controller could only
+	 * complete a partial write then the adjustment for the extra bytes at
+	 * the end is not needed.
+	 */
+	if (start + bytes_written == end)
+		ret -= end - (to + len);
+
+	/* Should not be possible. */
+	if (WARN_ON(ret < 0))
+		ret = -EIO;
+
+out:
+	kfree(tmp_buf);
+	return ret;
+}
+
 /*
  * Write an address range to the nor chip.  Data must be written in
  * FLASH_PAGESIZE chunks.  The address range may be any size provided
@@ -2067,7 +2133,12 @@ static int spi_nor_write(struct mtd_info *mtd, loff_t to, size_t len,
 		if (ret)
 			goto write_err;
 
-		ret = spi_nor_write_data(nor, addr, page_remain, buf + i);
+		if (nor->write_proto == SNOR_PROTO_8_8_8_DTR)
+			ret = spi_nor_octal_dtr_write(nor, addr, page_remain,
+						      buf + i);
+		else
+			ret = spi_nor_write_data(nor, addr, page_remain,
+						 buf + i);
 		if (ret < 0)
 			goto write_err;
 		written = ret;
-- 
2.30.0