The blockrom MTD driver provides bad block free read access to MTDs via /dev/blockromX device files. Bad blocks are automatically skipped upon read, and reading continues with the next good block.
This allows for read-only filesystems to exist in MTDs with bad blocks: usually the baddies are skipped when writing the filesystem into MTD by the tool performing the write (e.g. nandwrite). To successfully mount these filesystems, however, you will need a translation layer that skips bad blocks upon read as well. blockrom is such a translation layer.
Working with /dev/blockromX devices
Blockrom will create one device node per MTD - or, if MTD partitioning support was selected as well, it will create one node per MTD partition.
You cannot write to these devices. So in order to put something there your kernel will also require to support some means to write to MTDs; I recommend using the MTD character access driver for this.
Installing a file system
- First, erase the corresponding MTD (mtd3 in this example). The
flash_erasetool is from the
mtd-utilspackage of the linux-mtd project.
flash_erase /dev/mtd3 0 0
- Write the file system image (squashfs in this example) to the mtd char
device, skipping bad blocks as you write. Since this example writes to NAND
flash it uses
nandwrite, which also is from
nandwrite -m -p /dev/mtd3 fs-image.squashfs
- You can now mount the corresponding blockrom block device. This example assumes that you wrote a squashfs to mtd3 (like in the example above):
mount -t squashfs /dev/blockrom3 /mnt/mysquash
- copy the driver source into your kernel sources:
cp mtd-blockrom-ftl/src/drivers/mtd/* <where-your-kernel-is>/drivers/mtd
- apply the glue logic patch to your kernel
cd <where-your-kernel-is> patch -p1 < <where-you-downloaded-blockrom>/mtd-blockrom-glue.patch
make menuconfigor similar and select the blockrom ftl
make menuconfig drivers-> mtd-> [*] Bad-block-skipiping readonly 'blockrom' access to MTD devices
compile, deploy your kernel
Todo: Bit scrubbing
blockrom.c lacks of a handler for "corrected read errors". These happen e.g. if a one-bit CRC error occurred and was corrected. According to some posts at the linux-mtd mailing list thes errors may occur even if a flash device is only ever read from, never written to. Upon encountering such an error the driver is supposed to "bit scrub" the corresponding erase block. This basically means moving the EBs data some place else, and erasing the erroneous block. In order to safely support such an operation the blockrom driver needs to apply some kind of translation table when looking up block addresses and offsets.
There's a stub in the driver source where a bit scrubbing routine would go in, as well as a kernel config option for enabling it (if you enable it right now your kernel build would fail, though, since it's not yet implmented).
tests/ sub directory has unit tests for blockrom.c. You don't need a
kernel for building or running the tests, they're self-contained.
You do need
test_harness.h, a tiny collection of mocking tools for writing
C unit tests, though. You can obtain it from https://github.com/t-lo/test_harness.
Just put the .h file into the
tests/ sub directory and you should be good to
go. In order to build and run the tests just issue
make in the