2 files changed, 39 insertions, 9 deletions

diff --git a/drivers/mtd/ubi/Kconfig b/drivers/mtd/ubi/Kconfig
index b2f4f0f032f1..dcbaae3ead63 100644
--- a/drivers/mtd/ubi/Kconfig
+++ b/drivers/mtd/ubi/Kconfig
@@ -28,14 +28,29 @@ 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 eraseblock count per 1024 eraseblocks"
+        default 20
+        range 0 768
         help
           This option specifies the maximum bad physical eraseblocks UBI
-          expects on the UBI device (percents of total number of physical
-          eraseblocks on this MTD partition). If the underlying flash does not
-          admit of bad eraseblocks (e.g. NOR flash), this value is ignored.
+          expects on the MTD device (per 1024 eraseblocks). If the underlying
+          flash does not admit of bad eraseblocks (e.g. NOR flash), this value
+          is ignored.
+
+          NAND datasheets often specify the minimum and maximum NVM (Number of
+          Valid Blocks) for the flashes' endurance lifetime. The maximum
+          expected bad eraseblocks per 1024 eraseblocks then can be calculated
+          as "1024 * (1 - MinNVB / MaxNVB)", which gives 20 for most NANDs
+          (MaxNVB is basically the total count of eraseblocks on the chip).
+
+          To put it differently, if this value is 20, UBI will try to reserve
+          about 1.9% of physical eraseblocks for bad blocks handling. And that
+          will be 1.9% of eraseblocks on the entire NAND chip, not just the MTD
+          partition UBI attaches. This means that if you have, say, a NAND
+          flash chip admits maximum 40 bad eraseblocks, and it is split on two
+          MTD partitions of the same size, UBI will reserve 40 eraseblocks when
+          attaching a partition.
+
           Leave the default value if unsure.
 
 config MTD_UBI_GLUEBI
diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
index ce311aa75a75..9980441a8886 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>
@@ -610,11 +611,25 @@ 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 limit = mult_frac(ubi->peb_count, percent, 100);
+                        int per1024 = CONFIG_MTD_UBI_BEB_LIMIT;
+                        int limit, device_pebs;
+                        uint64_t device_size;
+
+                        /*
+                         * Here we are using size of the entire flash chip and
+                         * not just the MTD partition size because the maximum
+                         * number of bad eraseblocks is a percentage of the
+                         * whole device and bad eraseblocks are not fairly
+                         * distributed over the flash chip. So the worst case
+                         * is that all the bad eraseblocks of the chip are in
+                         * the MTD partition we are attaching (ubi->mtd).
+                         */
+                        device_size = mtd_get_device_size(ubi->mtd);
+                        device_pebs = mtd_div_by_eb(device_size, ubi->mtd);
+                        limit = mult_frac(device_pebs, per1024, 1024);
 
                         /* Round it up */
-                        if (mult_frac(limit, 100, percent) < ubi->peb_count)
+                        if (mult_frac(limit, 1024, per1024) < device_pebs)
                                 limit += 1;
                         ubi->bad_peb_limit = limit;
                 }