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authorArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2010-10-29 08:34:50 +0300
committerArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2011-02-06 19:19:41 +0200
commitfef2deb31f6523203a3fa1af485a5f1fef19cf6b (patch)
treed29f44eeb247b72329e0e16e19399dd4b83e5e26 /drivers/mtd
parent6c1e875ca6f3a47b40dce715bd07fdfdb8388d55 (diff)
downloadop-kernel-dev-fef2deb31f6523203a3fa1af485a5f1fef19cf6b.zip
op-kernel-dev-fef2deb31f6523203a3fa1af485a5f1fef19cf6b.tar.gz
UBI: cleanup comments about corrupted PEBs
Just make them a bit more readable and explanatory. Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Diffstat (limited to 'drivers/mtd')
-rw-r--r--drivers/mtd/ubi/scan.c54
1 files changed, 34 insertions, 20 deletions
diff --git a/drivers/mtd/ubi/scan.c b/drivers/mtd/ubi/scan.c
index 0028bf2..b65cc08 100644
--- a/drivers/mtd/ubi/scan.c
+++ b/drivers/mtd/ubi/scan.c
@@ -39,32 +39,46 @@
* eraseblocks are put to the @free list and the physical eraseblock to be
* erased are put to the @erase list.
*
+ * About corruptions
+ * ~~~~~~~~~~~~~~~~~
+ *
+ * UBI protects EC and VID headers with CRC-32 checksums, so it can detect
+ * whether the headers are corrupted or not. Sometimes UBI also protects the
+ * data with CRC-32, e.g., when it executes the atomic LEB change operation, or
+ * when it moves the contents of a PEB for wear-leveling purposes.
+ *
* UBI tries to distinguish between 2 types of corruptions.
- * 1. Corruptions caused by power cuts. These are harmless and expected
- * corruptions and UBI tries to handle them gracefully, without printing too
- * many warnings and error messages. The idea is that we do not lose
- * important data in these case - we may lose only the data which was being
- * written to the media just before the power cut happened, and the upper
- * layers (e.g., UBIFS) are supposed to handle these situations. UBI puts
- * these PEBs to the head of the @erase list and they are scheduled for
- * erasure.
+ *
+ * 1. Corruptions caused by power cuts. These are expected corruptions and UBI
+ * tries to handle them gracefully, without printing too many warnings and
+ * error messages. The idea is that we do not lose important data in these case
+ * - we may lose only the data which was being written to the media just before
+ * the power cut happened, and the upper layers (e.g., UBIFS) are supposed to
+ * handle such data losses (e.g., by using the FS journal).
+ *
+ * When UBI detects a corruption (CRC-32 mismatch) in a PEB, and it looks like
+ * the reason is a power cut, UBI puts this PEB to the @erase list, and all
+ * PEBs in the @erase list are scheduled for erasure later.
*
* 2. Unexpected corruptions which are not caused by power cuts. During
- * scanning, such PEBs are put to the @corr list and UBI preserves them.
- * Obviously, this lessens the amount of available PEBs, and if at some
- * point UBI runs out of free PEBs, it switches to R/O mode. UBI also loudly
- * informs about such PEBs every time the MTD device is attached.
+ * scanning, such PEBs are put to the @corr list and UBI preserves them.
+ * Obviously, this lessens the amount of available PEBs, and if at some point
+ * UBI runs out of free PEBs, it switches to R/O mode. UBI also loudly informs
+ * about such PEBs every time the MTD device is attached.
*
* However, it is difficult to reliably distinguish between these types of
- * corruptions and UBI's strategy is as follows. UBI assumes (2.) if the VID
- * header is corrupted and the data area does not contain all 0xFFs, and there
- * were not bit-flips or integrity errors while reading the data area. Otherwise
- * UBI assumes (1.). The assumptions are:
- * o if the data area contains only 0xFFs, there is no data, and it is safe
- * to just erase this PEB.
- * o if the data area has bit-flips and data integrity errors (ECC errors on
+ * corruptions and UBI's strategy is as follows. UBI assumes corruption type 2
+ * if the VID header is corrupted and the data area does not contain all 0xFFs,
+ * and there were no bit-flips or integrity errors while reading the data area.
+ * Otherwise UBI assumes corruption type 1. So the decision criteria are as
+ * follows.
+ * o If the data area contains only 0xFFs, there is no data, and it is safe
+ * to just erase this PEB - this is corruption type 1.
+ * o If the data area has bit-flips or data integrity errors (ECC errors on
* NAND), it is probably a PEB which was being erased when power cut
- * happened.
+ * happened, so this is corruption type 1. However, this is just a guess,
+ * which might be wrong.
+ * o Otherwise this it corruption type 2.
*/
#include <linux/err.h>
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