UBI: print a warning if too many PEBs are corrupted

There was a bug report recently where UBI prints:

UBI error: ubi_attach_mtd_dev: failed to attach by scanning, error -22

error messages and refuses to attach a PEB. It turned out to be a
buggy flash driver which returned garbage to almost every UBI read.
This patch makes UBI print a better message in such cases. Namely,
if UBI finds 8 or more corrupted PEBs, it prints a warning and
lists the corrupted PEBs.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/scan.c

@@ -75,9 +75,10 @@ static int add_to_list(struct ubi_scan_info *si, int pnum, int ec,
 		dbg_bld("add to free: PEB %d, EC %d", pnum, ec);
 	else if (list == &si->erase)
 		dbg_bld("add to erase: PEB %d, EC %d", pnum, ec);
-	else if (list == &si->corr)
+	else if (list == &si->corr) {
 		dbg_bld("add to corrupted: PEB %d, EC %d", pnum, ec);
-	else if (list == &si->alien)
+		si->corr_count += 1;
+	} else if (list == &si->alien)
 		dbg_bld("add to alien: PEB %d, EC %d", pnum, ec);
 	else
 		BUG();
@@ -937,6 +938,19 @@ struct ubi_scan_info *ubi_scan(struct ubi_device *ubi)
 	if (si->is_empty)
 		ubi_msg("empty MTD device detected");
 
+	/*
+	 * Few corrupted PEBs are not a problem and may be just a result of
+	 * unclean reboots. However, many of them may indicate some problems
+	 * with the flash HW or driver. Print a warning in this case.
+	 */
+	if (si->corr_count >= 8 || si->corr_count >= ubi->peb_count / 4) {
+		ubi_warn("%d PEBs are corrupted", si->corr_count);
+		printk(KERN_WARNING "corrupted PEBs are:");
+		list_for_each_entry(seb, &si->corr, u.list)
+			printk(KERN_CONT " %d", seb->pnum);
+		printk(KERN_CONT "\n");
+	}
+
 	/*
 	 * In case of unknown erase counter we use the mean erase counter
 	 * value.

drivers/mtd/ubi/scan.h

@@ -102,6 +102,7 @@ struct ubi_scan_volume {
  * @mean_ec: mean erase counter value
  * @ec_sum: a temporary variable used when calculating @mean_ec
  * @ec_count: a temporary variable used when calculating @mean_ec
+ * @corr_count: count of corrupted PEBs
  * @image_seq_set: indicates @ubi->image_seq is known
  *
  * This data structure contains the result of scanning and may be used by other
@@ -125,6 +126,7 @@ struct ubi_scan_info {
 	int mean_ec;
 	uint64_t ec_sum;
 	int ec_count;
+	int corr_count;
 	int image_seq_set;
 };