mtd: nand: omap: ecc.correct: omap_elm_correct_data: fix erased-page detection for BCHx_HW ECC schemes

As erased-pages do not have ECC stored in their OOB area, so they need to be
seperated out from programmed-pages, before doing BCH ECC correction.

In current implementation of omap_elm_correct_data() which does ECC correction
for BCHx ECC schemes, this erased-pages are detected based on specific marker
byte (reserved as 0x00) in ecc-layout.
However, this approach has some limitation like;
 1) All ecc-scheme layouts do not have such Reserved byte marker to
    differentiate between erased-page v/s programmed-page. Thus this is a
    customized solution.
 2) Reserved marker byte can itself be subjected to bit-flips causing
    erased-page to be misunderstood as programmed-page.

This patch removes dependency on any marker byte in ecc-layout, instead it
compares calc_ecc[] with pattern of ECC-of-all(0xff). This implicitely
means that both 'data + oob == all(0xff).

Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/omap2.c

@@ -1338,21 +1338,8 @@ static int erased_sector_bitflips(u_char *data, u_char *oob,
  * @calc_ecc:	ecc read from HW ECC registers
  *
  * Calculated ecc vector reported as zero in case of non-error pages.
- * In case of error/erased pages non-zero error vector is reported.
+ * In case of non-zero ecc vector, first filter out erased-pages, and
- * In case of non-zero ecc vector, check read_ecc at fixed offset
+ * then process data via ELM to detect bit-flips.
- * (x = 13/7 in case of BCH8/4 == 0) to find page programmed or not.
- * To handle bit flips in this data, count the number of 0's in
- * read_ecc[x] and check if it greater than 4. If it is less, it is
- * programmed page, else erased page.
- *
- * 1. If page is erased, check with standard ecc vector (ecc vector
- * for erased page to find any bit flip). If check fails, bit flip
- * is present in erased page. Count the bit flips in erased page and
- * if it falls under correctable level, report page with 0xFF and
- * update the correctable bit information.
- * 2. If error is reported on programmed page, update elm error
- * vector and correct the page with ELM error correction routine.
- *
  */
 static int omap_elm_correct_data(struct mtd_info *mtd, u_char *data,
 				u_char *read_ecc, u_char *calc_ecc)
@@ -1367,6 +1354,8 @@ static int omap_elm_correct_data(struct mtd_info *mtd, u_char *data,
 	u_char *ecc_vec = calc_ecc;
 	u_char *spare_ecc = read_ecc;
 	u_char *erased_ecc_vec;
+	u_char *buf;
+	int bitflip_count;
 	enum bch_ecc type;
 	bool is_error_reported = false;
 
@@ -1374,10 +1363,12 @@ static int omap_elm_correct_data(struct mtd_info *mtd, u_char *data,
 	case OMAP_ECC_BCH4_CODE_HW:
 		/* omit  7th ECC byte reserved for ROM code compatibility */
 		actual_eccbytes = ecc->bytes - 1;
+		erased_ecc_vec = bch4_vector;
 		break;
 	case OMAP_ECC_BCH8_CODE_HW:
 		/* omit 14th ECC byte reserved for ROM code compatibility */
 		actual_eccbytes = ecc->bytes - 1;
+		erased_ecc_vec = bch8_vector;
 		break;
 	default:
 		pr_err("invalid driver configuration\n");
@@ -1389,10 +1380,8 @@ static int omap_elm_correct_data(struct mtd_info *mtd, u_char *data,
 
 	if (info->nand.ecc.strength == BCH8_MAX_ERROR) {
 		type = BCH8_ECC;
-		erased_ecc_vec = bch8_vector;
 	} else {
 		type = BCH4_ECC;
-		erased_ecc_vec = bch4_vector;
 	}
 
 	for (i = 0; i < eccsteps ; i++) {
@@ -1410,44 +1399,36 @@ static int omap_elm_correct_data(struct mtd_info *mtd, u_char *data,
 		}
 
 		if (eccflag == 1) {
-			/*
+			if (memcmp(calc_ecc, erased_ecc_vec,
-			 * Set threshold to minimum of 4, half of ecc.strength/2
+						actual_eccbytes) == 0) {
-			 * to allow max bit flip in byte to 4
-			 */
-			unsigned int threshold = min_t(unsigned int, 4,
-					info->nand.ecc.strength / 2);
-
-			/*
-			 * Check data area is programmed by counting
-			 * number of 0's at fixed offset in spare area.
-			 * Checking count of 0's against threshold.
-			 * In case programmed page expects at least threshold
-			 * zeros in byte.
-			 * If zeros are less than threshold for programmed page/
-			 * zeros are more than threshold erased page, either
-			 * case page reported as uncorrectable.
-			 */
-			if (hweight8(~read_ecc[actual_eccbytes]) >= threshold) {
 				/*
-				 * Update elm error vector as
+				 * calc_ecc[] matches pattern for ECC(all 0xff)
-				 * data area is programmed
+				 * so this is definitely an erased-page
 				 */
-				err_vec[i].error_reported = true;
-				is_error_reported = true;
 			} else {
-				/* Error reported in erased page */
+				buf = &data[info->nand.ecc.size * i];
-				int bitflip_count;
+				/*
-				u_char *buf = &data[info->nand.ecc.size * i];
+				 * count number of 0-bits in read_buf.
-
+				 * This check can be removed once a similar
-				if (memcmp(calc_ecc, erased_ecc_vec,
+				 * check is introduced in generic NAND driver
-							 actual_eccbytes)) {
+				 */
-					bitflip_count = erased_sector_bitflips(
+				bitflip_count = erased_sector_bitflips(
-							buf, read_ecc, info);
+						buf, read_ecc, info);
-
+				if (bitflip_count) {
-					if (bitflip_count)
+					/*
-						stat += bitflip_count;
+					 * number of 0-bits within ECC limits
-					else
+					 * So this may be an erased-page
-						return -EINVAL;
+					 */
+					stat += bitflip_count;
+				} else {
+					/*
+					 * Too many 0-bits. It may be a
+					 * - programmed-page, OR
+					 * - erased-page with many bit-flips
+					 * So this page requires check by ELM
+					 */
+					err_vec[i].error_reported = true;
+					is_error_reported = true;
 				}
 			}
 		}