[coreboot] how to implement variable read/write variable in coreboot

[Felix Held]

Hi Nico!

>> The question I have here is what to do if there is more than one valid
>> entry. Just using the first one introduces a bit of undetermined
>> behavior (block order in the flash might be different after
>> defragmentation).
> No, always choosing the first would be determined behavior. In case we
> keep things in order it's also the better choice when it comes to sear-
> ching the current, valid entry (we'd have to always search the whole
> used space for a newer version otherwise). I'd treat it like this: If
> the old version wasn't invalidated, the update didn't finish. So we can
> ignore the new version.
I was assuming the ring buffer case here where the first entry in the 
raw flash address space doesn't have to be the oldest one.

>> So either use some counter (beware of overflows!) on
>> either the flash erase block (small overhead and rather efficient) or
>> the entry (full region has to be either cached or read every time and
>> needs more bytes than the counter per erase block) or accept the
>> possibility of undetermined behavior (IMHO not a good idea).
> I'm not sure how to use a per erase-block counter? When would it update?
> we could only do that on every erase. A per variable counter might be
> useful, but I still hope it will work without.
I was thinking of putting a number in the first word of a block. Every 
time a new block is allocated, this value is written once. The value is 
the highest number already used plus 1. Also beware of wraparounds there.

> Might be done in a sanitation phase before each write. But if we define
> the first entry to be the valid one, we probably don't need it. Update:
> I didn't come around in my scheme below... maybe we should make the
> update in three steps: tag for update, insert new value, invalidate.
> This way we would only have to search for duplicates for valid entries
> tagged for update.
Good idea.

> Defragmentation:
Haven't really looked at and thought about your defragmentation phase 
idea; maybe I'll comment on that next week.

Regards
Felix