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    Subject[PATCH 3/4] UBI: use the whole MTD device size to get bad_peb_limit
    Date
    On NAND flash devices, UBI reserves some physical erase blocks (PEB) for
    bad block handling.
    Today, the number of reserved PEB can only be set as a percentage of the
    total number of PEB in each MTD partition.
    For example, for a NAND flash with 128KiB PEB, 2 MTD partition of 20MiB
    (mtd0) and 100MiB (mtd1) and 2% reserved PEB:
    - the UBI device on mtd0 will have 2 PEB reserved
    - the UBI device on mtd1 will have 16 PEB reserved

    The problem with this behaviour is that NAND flash manufacturers give a
    minimum number of valid block (NVB) during the endurance life of the
    device.
    E.G.:
    Parameter Symbol Min Max Unit Notes
    --------------------------------------------------------------
    Valid block number NVB 1004 1024 Blocks 1
    Note:
    1. Invalid blocks are block that contain one or more bad bits beyond
    ECC. The device may contain bad blocks upon shipment. Additional bad
    blocks may develop over time; however, the total number of available
    blocks will not drop below NVB during the endurance life of the device.

    From this number we can deduce the maximum number of bad PEB that a
    device will contain during its endurance life :
    A 128MiB NAND flash (1024 PEB) will not have less than 20 bad blocks
    during the flash endurance life.

    BUT, the manufacturer doesn't tell where those bad block will appear. He
    doesn't say either if they will be equally disposed on the whole device
    (and I'm pretty sure they won't).
    So, according to the datasheets, we should reserve the maximum number of
    bad PEB for each UBI device.
    (Worst case scenario: 20 bad blocks appears on the smallest MTD
    partition.)

    So this patch make UBI use the whole MTD device size to calculate the
    Maximum bad expected eraseblocks.

    The Kconfig option is in per1024 blocks, thus it can have a default
    value of 20 which is *very* common for NAND devices.

    Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
    ---
    drivers/mtd/ubi/Kconfig | 14 ++++++++++----
    drivers/mtd/ubi/build.c | 17 ++++++++++++++---
    2 files changed, 24 insertions(+), 7 deletions(-)

    diff --git a/drivers/mtd/ubi/Kconfig b/drivers/mtd/ubi/Kconfig
    index 145cda2..b77ffe1 100644
    --- a/drivers/mtd/ubi/Kconfig
    +++ b/drivers/mtd/ubi/Kconfig
    @@ -28,15 +28,21 @@ config MTD_UBI_WL_THRESHOLD
    to 128 or 256, although it does not have to be power of 2).

    config MTD_UBI_BEB_LIMIT
    - int "Percentage of maximum expected bad eraseblocks"
    - default 2
    - range 0 25
    + int "Maximum expected bad eraseblocks per 1024 eraseblocks"
    + default 20
    + range 2 256
    help
    If the MTD device admits of bad eraseblocks (e.g. NAND flash), UBI
    reserves some amount of physical eraseblocks to handle new bad
    eraseblocks.
    This option specifies the maximum bad eraseblocks UBI expects on the
    - ubi device (percents of total number of flash eraseblocks).
    + ubi device per 1024 eraseblocks.
    + This value is often given in an other form in the NAND datasheet
    + (min NVB i.e. minimal number of valid blocks). The maximum expected
    + bad eraseblocks per 1024 is then:
    + 1024 * (1 - MinNVB / MaxNVB)
    + Which gives 20 for most NAND devices.
    +
    This limit is used in order to derive amount of eraseblock UBI
    reserves for handling new bad blocks.
    If the device has more bad eraseblocks than this limit, UBI does not
    diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
    index 62cc6ce..87b39c2 100644
    --- a/drivers/mtd/ubi/build.c
    +++ b/drivers/mtd/ubi/build.c
    @@ -36,6 +36,7 @@
    #include <linux/namei.h>
    #include <linux/stat.h>
    #include <linux/miscdevice.h>
    +#include <linux/mtd/partitions.h>
    #include <linux/log2.h>
    #include <linux/kthread.h>
    #include <linux/kernel.h>
    @@ -610,12 +611,22 @@ static int io_init(struct ubi_device *ubi)
    if (mtd_can_have_bb(ubi->mtd)) {
    ubi->bad_allowed = 1;
    if (CONFIG_MTD_UBI_BEB_LIMIT > 0) {
    - int percent = CONFIG_MTD_UBI_BEB_LIMIT;
    + int per1024 = CONFIG_MTD_UBI_BEB_LIMIT;
    + int device_peb_count;
    + uint64_t device_size;
    int beb_limit;

    - beb_limit = mult_frac(ubi->peb_count, percent, 100);
    + /* we are using here the whole MTD device size and not
    + * the MTD partition size because the maximum number of
    + * bad blocks is a percentage of the whole device and
    + * the bad blocks are not fairly disposed on a flash
    + * device
    + */
    + device_size = mtd_get_device_size(ubi->mtd);
    + device_peb_count = mtd_div_by_eb(device_size, ubi->mtd);
    + beb_limit = mult_frac(device_peb_count, per1024, 1024);
    /* round it up */
    - if (mult_frac(beb_limit, 100, percent) < ubi->peb_count)
    + if (mult_frac(beb_limit, 1024, per1024) < ubi->peb_count)
    beb_limit++;
    ubi->bad_peb_limit = beb_limit;
    }
    --
    1.7.2.5


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