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authorArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2010-10-29 01:34:50 -0400
committerArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2011-02-06 12:19:41 -0500
commitfef2deb31f6523203a3fa1af485a5f1fef19cf6b (patch)
treed29f44eeb247b72329e0e16e19399dd4b83e5e26 /drivers/mtd
parent6c1e875ca6f3a47b40dce715bd07fdfdb8388d55 (diff)
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 0028bf283930..b65cc088fde5 100644
--- a/drivers/mtd/ubi/scan.c
+++ b/drivers/mtd/ubi/scan.c
@@ -39,32 +39,46 @@
39 * eraseblocks are put to the @free list and the physical eraseblock to be 39 * eraseblocks are put to the @free list and the physical eraseblock to be
40 * erased are put to the @erase list. 40 * erased are put to the @erase list.
41 * 41 *
42 * About corruptions
43 * ~~~~~~~~~~~~~~~~~
44 *
45 * UBI protects EC and VID headers with CRC-32 checksums, so it can detect
46 * whether the headers are corrupted or not. Sometimes UBI also protects the
47 * data with CRC-32, e.g., when it executes the atomic LEB change operation, or
48 * when it moves the contents of a PEB for wear-leveling purposes.
49 *
42 * UBI tries to distinguish between 2 types of corruptions. 50 * UBI tries to distinguish between 2 types of corruptions.
43 * 1. Corruptions caused by power cuts. These are harmless and expected 51 *
44 * corruptions and UBI tries to handle them gracefully, without printing too 52 * 1. Corruptions caused by power cuts. These are expected corruptions and UBI
45 * many warnings and error messages. The idea is that we do not lose 53 * tries to handle them gracefully, without printing too many warnings and
46 * important data in these case - we may lose only the data which was being 54 * error messages. The idea is that we do not lose important data in these case
47 * written to the media just before the power cut happened, and the upper 55 * - we may lose only the data which was being written to the media just before
48 * layers (e.g., UBIFS) are supposed to handle these situations. UBI puts 56 * the power cut happened, and the upper layers (e.g., UBIFS) are supposed to
49 * these PEBs to the head of the @erase list and they are scheduled for 57 * handle such data losses (e.g., by using the FS journal).
50 * erasure. 58 *
59 * When UBI detects a corruption (CRC-32 mismatch) in a PEB, and it looks like
60 * the reason is a power cut, UBI puts this PEB to the @erase list, and all
61 * PEBs in the @erase list are scheduled for erasure later.
51 * 62 *
52 * 2. Unexpected corruptions which are not caused by power cuts. During 63 * 2. Unexpected corruptions which are not caused by power cuts. During
53 * scanning, such PEBs are put to the @corr list and UBI preserves them. 64 * scanning, such PEBs are put to the @corr list and UBI preserves them.
54 * Obviously, this lessens the amount of available PEBs, and if at some 65 * Obviously, this lessens the amount of available PEBs, and if at some point
55 * point UBI runs out of free PEBs, it switches to R/O mode. UBI also loudly 66 * UBI runs out of free PEBs, it switches to R/O mode. UBI also loudly informs
56 * informs about such PEBs every time the MTD device is attached. 67 * about such PEBs every time the MTD device is attached.
57 * 68 *
58 * However, it is difficult to reliably distinguish between these types of 69 * However, it is difficult to reliably distinguish between these types of
59 * corruptions and UBI's strategy is as follows. UBI assumes (2.) if the VID 70 * corruptions and UBI's strategy is as follows. UBI assumes corruption type 2
60 * header is corrupted and the data area does not contain all 0xFFs, and there 71 * if the VID header is corrupted and the data area does not contain all 0xFFs,
61 * were not bit-flips or integrity errors while reading the data area. Otherwise 72 * and there were no bit-flips or integrity errors while reading the data area.
62 * UBI assumes (1.). The assumptions are: 73 * Otherwise UBI assumes corruption type 1. So the decision criteria are as
63 * o if the data area contains only 0xFFs, there is no data, and it is safe 74 * follows.
64 * to just erase this PEB. 75 * o If the data area contains only 0xFFs, there is no data, and it is safe
65 * o if the data area has bit-flips and data integrity errors (ECC errors on 76 * to just erase this PEB - this is corruption type 1.
77 * o If the data area has bit-flips or data integrity errors (ECC errors on
66 * NAND), it is probably a PEB which was being erased when power cut 78 * NAND), it is probably a PEB which was being erased when power cut
67 * happened. 79 * happened, so this is corruption type 1. However, this is just a guess,
80 * which might be wrong.
81 * o Otherwise this it corruption type 2.
68 */ 82 */
69 83
70#include <linux/err.h> 84#include <linux/err.h>