[U-Boot] Question regarding NAND environment

[Peter Barada <peter.barada@logicpd.com>]

On 01/27/2012 04:46 PM, Scott Wood wrote:
> 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.
I'm probalby being paranoid, but from what I've seen, if it can fail,
odds are it will.

I can add code to mark the blocks bad if the erase/write fails.

>> 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).
Its somewhere in the middle.  REDUND give you two copies.  ENV_RANGE
gives you one copy but allows it to
live in the first good block.  Modifying either way is going to affect
units in the field.  But I'll give it a whirl.
> Probably better to never update the environment in the field -- source a
> script in an ubi partition instead.
Proper planning will save one from having to update the environment in
the field, but I'm sure it happens.  I just figured I'd try to make sure
that nothing bad happens when people do...




-- 
Peter Barada
peter.barada at logicpd.com