Wolfgang Denk wrote:
Dear Jerry Van Baren,
In message 49E335BB.4050609@ge.com you wrote:
- it works only on NOR flash, not on any media that can be accessed only in blocks
Not true: the offset merely needs to be aligned to the next block in that case: <next><crc><env><NUL>FFFFFFFFFFFF <next><crc><env><NUL>[undefined] ^^^^ offset of ------------------> ^^^^ 0xFFFFFFFF
All flash that I can recall seeing allows at least a limited number of rewrites to a block (the limited number getting smaller on successive generations), so two writes to the env start block (once to write the new env, a second time to rewrite the 0xFFFFFFFF to point to the superceeding env). If the flash didn't support two writes, that would be a show-stopper. Due to how people use flash, I don't think the number will ever go to one write (IIRC, 4 is typical of current generation NAND).
So what do we save, then? Instead of a single write we now need actually two...
Well, I have a (NOR) flash with a 128Kbyte sector size. If my env is ~2K, I can do ~64 updates before I would need to erase the sector and start over.
From a practical point of view, that probably isn't a big deal. From a "bugs me" POV, it bugs me that I'm "wasting" 126K of that sector. :-P
IIRC, redundant env lumps take two sectors so the redundant env isn't erased (need to check the code). Depending on your acceptable risk tolerance, the "redundant" env lump would simply be lump(n-1).
[snip]
Best regards, gvb