mtd: spi-nor: core: avoid odd length/address reads on 8D-8D-8D mode

On Octal DTR capable flashes like Micron Xcella reads cannot start or
end at an odd address in Octal DTR mode. Extra bytes need to be read at
the start or end to make sure both the start address and length remain
even.

To avoid allocating too much extra memory, thereby putting unnecessary
memory pressure on the system, the temporary buffer containing the extra
padding bytes is capped at PAGE_SIZE bytes. The rest of the 2-byte
aligned part should be read directly in the main buffer.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Luke Wang <ziniu.wang_1@nxp.com>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20250708091646.292-1-ziniu.wang_1@nxp.com

Author: Pratyush Yadav

drivers/mtd/spi-nor/core.c

@@ -2014,6 +2014,76 @@ static const struct flash_info *spi_nor_detect(struct spi_nor *nor)
 	return info;
 }
 
+/*
+ * On Octal DTR capable flashes, reads cannot start or end at an odd
+ * address in Octal DTR mode. Extra bytes need to be read at the start
+ * or end to make sure both the start address and length remain even.
+ */
+static int spi_nor_octal_dtr_read(struct spi_nor *nor, loff_t from, size_t len,
+				  u_char *buf)
+{
+	u_char *tmp_buf;
+	size_t tmp_len;
+	loff_t start, end;
+	int ret, bytes_read;
+
+	if (IS_ALIGNED(from, 2) && IS_ALIGNED(len, 2))
+		return spi_nor_read_data(nor, from, len, buf);
+	else if (IS_ALIGNED(from, 2) && len > PAGE_SIZE)
+		return spi_nor_read_data(nor, from, round_down(len, PAGE_SIZE),
+					 buf);
+
+	tmp_buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!tmp_buf)
+		return -ENOMEM;
+
+	start = round_down(from, 2);
+	end = round_up(from + len, 2);
+
+	/*
+	 * Avoid allocating too much memory. The requested read length might be
+	 * quite large. Allocating a buffer just as large (slightly bigger, in
+	 * fact) would put unnecessary memory pressure on the system.
+	 *
+	 * For example if the read is from 3 to 1M, then this will read from 2
+	 * to 4098. The reads from 4098 to 1M will then not need a temporary
+	 * buffer so they can proceed as normal.
+	 */
+	tmp_len = min_t(size_t, end - start, PAGE_SIZE);
+
+	ret = spi_nor_read_data(nor, start, tmp_len, tmp_buf);
+	if (ret == 0) {
+		ret = -EIO;
+		goto out;
+	}
+	if (ret < 0)
+		goto out;
+
+	/*
+	 * More bytes are read 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 read.
+	 */
+	bytes_read = ret;
+
+	if (from != start)
+		ret -= from - start;
+
+	/*
+	 * Only account for extra bytes at the end if they were actually read.
+	 * For example, if the total length was truncated because of temporary
+	 * buffer size limit then the adjustment for the extra bytes at the end
+	 * is not needed.
+	 */
+	if (start + bytes_read == end)
+		ret -= end - (from + len);
+
+	memcpy(buf, tmp_buf + (from - start), ret);
+out:
+	kfree(tmp_buf);
+	return ret;
+}
+
 static int spi_nor_read(struct mtd_info *mtd, loff_t from, size_t len,
 			size_t *retlen, u_char *buf)
 {
@@ -2031,7 +2101,11 @@ static int spi_nor_read(struct mtd_info *mtd, loff_t from, size_t len,
 	while (len) {
 		loff_t addr = from;
 
-		ret = spi_nor_read_data(nor, addr, len, buf);
+		if (nor->read_proto == SNOR_PROTO_8_8_8_DTR)
+			ret = spi_nor_octal_dtr_read(nor, addr, len, buf);
+		else
+			ret = spi_nor_read_data(nor, addr, len, buf);
+
 		if (ret == 0) {
 			/* We shouldn't see 0-length reads */
 			ret = -EIO;