mtd: rawnand: qcom: fix null pointer access for erased page detection

parse_read_errors can be called with only oob_buf in which case
data_buf will be NULL.  If data_buf is NULL, then don’t
treat this page as completely erased in case of ECC uncorrectable
error for RS ECC. For BCH ECC, the controller itself tells
regarding erased page in status register.
Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

drivers/mtd/nand/raw/qcom_nandc.c

@@ -1611,13 +1611,24 @@ static int parse_read_errors(struct qcom_nand_host *host, u8 *data_buf,
 			int ret, ecclen, extraooblen;
 			void *eccbuf;
 
-			/* ignore erased codeword errors */
+			/*
+			 * For BCH ECC, ignore erased codeword errors, if
+			 * ERASED_CW bits are set.
+			 */
 			if (host->bch_enabled) {
 				erased = (erased_cw & ERASED_CW) == ERASED_CW ?
 					 true : false;
-			} else {
+			/*
+			 * For RS ECC, HW reports the erased CW by placing
+			 * special characters at certain offsets in the buffer.
+			 * These special characters will be valid only if
+			 * complete page is read i.e. data_buf is not NULL.
+			 */
+			} else if (data_buf) {
 				erased = erased_chunk_check_and_fixup(data_buf,
 								      data_len);
+			} else {
+				erased = false;
 			}
 
 			if (erased) {
@@ -1665,7 +1676,8 @@ static int parse_read_errors(struct qcom_nand_host *host, u8 *data_buf,
 			max_bitflips = max(max_bitflips, stat);
 		}
 
-		data_buf += data_len;
+		if (data_buf)
+			data_buf += data_len;
 		if (oob_buf)
 			oob_buf += oob_len + ecc->bytes;
 	}