[U-Boot] Question regarding NAND environment

[Scott Wood <scottwood@freescale.com>]

On 01/26/2012 11:34 AM, Peter Barada wrote:
> On 01/26/2012 12:27 PM, Scott Wood wrote:
>> Why are two copies insufficient for that?
> Two copies are sufficient, if none of the blocks ever go bad.
> 
> To simplify things, suppose the environment is the same size as a block
> and you have only two blocks (and two copies) to hold the environment. 
> If one block goes bad then there is a window between when the one
> remaining block is erased and written with the environment that if power
> fails then there is no environment in NAND.

It seems unlikely, but possible I guess.  Currently I don't think we
dynamically mark blocks bad at all in U-Boot, except in things like ubi
and yaffs.

> To solve this I can crank up the number of blocks to three which allows
> one block to go bad and still at all times have one good copy of the
> environment in NAND.  But looking at writeenv(), it stops as soon as
> either nand_write fails, or one copy of the environment is written. So
> it could make sense to modify writeenv to write as many copies of the
> environment that fit into CONFIG_ENV_RANGE, and have readenv read out
> copies and verify them until it finds one good one.

This isn't what CONFIG_ENV_RANGE is about.  I think it would make more
sense to change REDUND to support more than two copies (each with their
own range).

Probably better to never update the environment in the field -- source a
script in an ubi partition instead.

-Scott