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

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>

diff --git a/drivers/mtd/nand/omap2.c b/drivers/mtd/nand/omap2.c
index 4bf2f76..a6c979f 100644 (file)
--- a/drivers/mtd/nand/omap2.c
+++ b/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, check read_ecc at fixed offset
- * (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.
- *
+ * In case of non-zero ecc vector, first filter out erased-pages, and
+ * then process data via ELM to detect bit-flips.
  */
 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) {
-                       /*
-                        * Set threshold to minimum of 4, half of ecc.strength/2
-                        * 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) {
+                       if (memcmp(calc_ecc, erased_ecc_vec,
+                                               actual_eccbytes) == 0) {
                                /*
-                                * Update elm error vector as
-                                * data area is programmed
+                                * calc_ecc[] matches pattern for ECC(all 0xff)
+                                * so this is definitely an erased-page
                                 */
-                               err_vec[i].error_reported = true;
-                               is_error_reported = true;
                        } else {
-                               /* Error reported in erased page */
-                               int bitflip_count;
-                               u_char *buf = &data[info->nand.ecc.size * i];
-
-                               if (memcmp(calc_ecc, erased_ecc_vec,
-                                                        actual_eccbytes)) {
-                                       bitflip_count = erased_sector_bitflips(
-                                                       buf, read_ecc, info);
-
-                                       if (bitflip_count)
-                                               stat += bitflip_count;
-                                       else
-                                               return -EINVAL;
+                               buf = &data[info->nand.ecc.size * i];
+                               /*
+                                * count number of 0-bits in read_buf.
+                                * This check can be removed once a similar
+                                * check is introduced in generic NAND driver
+                                */
+                               bitflip_count = erased_sector_bitflips(
+                                               buf, read_ecc, info);
+                               if (bitflip_count) {
+                                       /*
+                                        * number of 0-bits within ECC limits
+                                        * So this may be an erased-page
+                                        */
+                                       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;
                                }
                        }
                }