Lanchon
commented
3 months ago hi @yfhyou,
Is there a 'simple' way for this device?
no.
I tried searching forums and OpenWRT wiki
to my knowledge, how to restore stock on this device was never discussed. in this repo i provided a way for people to backup before wiping their device, so that at least there would be the possibility of restoring stock if needed, but i never outlined a procedure for restoring.
the easiest way to restore stock on any device is:
- install/boot an OS that runs entirely from ram.
- use that OS to restore the flash.
openwrt initramfs builds run entirely from ram (the OS is contained in a ramdrive that is bundled with the kernel) and are a perfect fit for this purpose when available. they are for this device, so this point is covered.
but the restore part can be trickier.
WARNING: if the restore is started but not correctly finished (eg, powerloss or incorrect procedure), the device may not boot at all and recovery methods may be needed (eg, bootloader access).
if the device is eMMC, restoring the partitions is trivial. on NAND devices such as this one, it is a bit more complicated. NANDs can contain bad blocks and new bad blocks can develop at any moment. and these bad blocks are user visible and not hidden by a translation layer as in SSDs. (bad blocks typically develop during erase or write, but not read, and this is what the software expects. however, NANDs also can and will "forget" the data written to them given enough time, without the associated blocks being bad in any way. this effect is generally not compensated for in openwrt, other than by having two copies of the firmware in some devices.)
so you technically cannot in the general case "restore" a NAND to a previous state, as the NAND may have developed bad blocks since that previous time. what you can do is restore the NAND to a state that will look indistinguishable from the original state to the applicable software that will be reading the NAND. this concept is key: how to restore a NAND partition depends on how it will be used by the software. there is no general method as there is for block devices such as SSDs and eMMCs.
openwrt can use a NAND partition in 2 ways (that i can think of):
- the partition is used as a single read-only (technically append-only), non-random access file. the partition can be wiped and completely rewritten from the start, but that is the only way to modify the partition. (technically you could append to it too, but that option is not used in openwrt.) writes proceed from beginning to end, skipping bad blocks. each block is verified immediately after writing, and declared bad and skipped if the write failed. reads proceed from beginning to end, skipping bad blocks. random access is not possible because you do not know how many bad blocks lie before your random access starting point. (but the kernel has a mechanism to provide random access called
mtdblock: it involves reading the partition sequentially and keeping a list of bad blocks in ram.) openwrt uses partitions in this way to store kernels and read-only squashfs rootfs filesystems. in openwrt you write these partitions using themtdcommand. - the partition is used as an UBI partition, which can contain UBI volumes, which can contain single files or filesystems. the UBI layer takes care of most complexities in NAND and gives you abstractions called UBI volumes which are far easier to use than NAND directly. openwrt uses UBI volumes to store kernels, read-only squashfs rootfs filesystems, and read-write UBIFS rootfs_data filesystems. in openwrt you restore complete UBI partitions (ie, with all their contained volumes) using the
ubiformat --flash-image=<file>command.
(openwrt also supports NOR flash, which is not expected to develop defects during its lifetime and thus is much easier to use. NOR partitions can be used directly for filesystems such as jffs2. but i will not cover NOR flash here.)
regarding your device, in official openwrt this device uses a big UBI partition storing kernel, rootfs, and rootfs_data volumes. (this is known as a kernel-in-UBI configuration.) see the output of ubinfo -a for more info.
(@hauke @mans0n @dangowrt, please read from this point on, if you have the time.)
so it would seem that return to stock is simple: just boot an initramfs build, scp the restore images to /tmp, ssh and write the images to flash using the right command from above, and finally reboot.
note that before reboot you would also need to restore the u-boot environment (see fw_printenv), which can be done by restoring the partition in which it is stored (if you had the backup), or manually using fw_setenv commands.
but that's it. it's simple, right? right?
err... unfortunately, nope.
this device as stock has partitions for two copies of its firmware. the people who ported this device repartitioned it: they deleted several stock partitions and created new, fewer, and larger partitions to store a single copy of openwrt and its non-volatile state.
and the problem is: when you run the initramfs official openwrt image, the partitions you want to restore simply don't exist! the "easiest" way to restore, IMHO, would be to setup an openwrt build environment and build your own modified initramfs kernel with the original partition table in it.
this is hardly easy or simple, even for experienced users. i have been advocating for a solution to this problem for some time now. take a look, this is a similar device i have ported (NAND, dual firmware stock, single firmware official openwrt):
for that device i have:
- kept all stock partitions (the ones i would have otherwise deleted), prepending
stock_to their names. - marked all such partitions as read-write to be able to use them to restore stock.
- and even made sure that all these partitions retained their original MTD numbers (which means that newly added partitions by me come after all stock partitions in the partition table, even if they are physically scattered around in the flash).
if this is done and enforced in all new ports, restoring say stock partition mtd26 is a simple affair of writing to /dev/mtd26 in official openwrt. this makes backup and restore scripts work unmodified both in stock rom and official openwrt.
as said, i have been advocating for this convention for a while, but for reasons that were not explained to me the project does not seem to favor this idea. or maybe they were too busy? idk.
back to your particular device... this convention was not followed, so restoring stock on this device is hard. not impossible though. but i would not recommend trying unless you have working access to the bootloader/serial port.
yfhyou
How would one go about restoring the mtd / ubi backups?
I tried searching forums and OpenWRT wiki, but I am quickly overwhelmed with all the options with NAND, NOR, dd, mtdwrite, etc etc.
Is there a 'simple' way for this device?