UBIFS on write-protected NAND

Richard Weinberger richard at nod.at
Mon Jun 3 05:58:10 PDT 2019


----- Ursprüngliche Mail -----
> Your HW engineers are wrong and did not read and _understand_ the NAND
> datasheets. Nor do they understand the software and what it does. The
> days of the HW guy designing something and throwing it over the wall
> and asking the SW guy to make it work are long over.
> 
> If you want NAND to stay bootable "despite everything in the world",
> you can't run it write protected. NAND will bit-rot over time. It is
> the nature of NAND. UBIFS detects this and will move data around as
> necessary to keep it readable. There are certain areas that really
> only get read at boot time, so if it's write protected at that point,
> you're sunk - UBIFS can't do the work of preserving the NAND that it
> is designed to do.
> 
> If it were me, in u-boot (or whatever bootloader you're using), I'd
> flip the GPIO holding the /WP line to make the NAND writable before I
> booted the kernel and then I'd leave it there.
> 
> The other way requires more effort - you could go into your NAND
> driver, find the low-level write sequences and flip the GPIO to write
> and close it to protect after you're done. But, pay very close
> attention to your datasheets to be sure you have your setup and hold
> times correct if you're going to do that.
> 
> The final way to do it is to not use UBIFS at all.  Run a R/O image
> like squashfs and run the NAND with way higher ECC than required and
> hope that over the lifetime of the device you don't accumulate more
> than that bit-flips in any sector that you care about.

Amen. :-)

Thanks,
//richard



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