mtd-blockrom-ftl

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_erase tool is from the mtd-utils package 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 mtd-utils of linux-mtd.

    nandwrite -m -p /dev/mtd3 fs-image.squashfs

Mouning blockrom

• 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

Installation

• 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

• run make menuconfig or similar and select the blockrom ftl

    make menuconfig
    drivers->
           mtd->
              [*] Bad-block-skipiping readonly 'blockrom' access to MTD devices

• compile, deploy your kernel

• done

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

The 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 tests/ sub directory.