Wolfgang Denk wrote:
Dear Tolunay,
in message 445B8086.9000404@orkun.us you wrote:
This patch would solve the issue that exists today that when the "active" environment is lost/corrupted for some reason the "redundant" environment would contain an exact copy of the primary to have the board come up without requiring the need to redo the changes that was lost on
Actually I think that you will not acchieve this with your patch. This is why I'm concerned. You see, if you feel better having this patch I would not complain, but I am afraid that a lot of people might just activate it because they think it would do them any good when it doesn't (and actually it just hurts).
There is only one occasion when we have any significant likelyhood of losing the environment data: this is when a call to "saveenv" fails becaue either a) we have a power loss, b) we have an otherwise induced reset of the CPU, or c) the flash sector that shall be erased/written is failing.
So where exactly does your modification improve anything? Let's go through this step by step.
Case 1: power loss/reset happens during the first "saveenv", i. e. when writing the first copy of the new environment data.
In this case this first copy contains no valid data; the second copy of the environment contains valid, but old data. This is exactly the same as we have with the current imple- mentation. I don't see any improvement.Case 2: power loss/reset happens during the second "saveenv", i. e. when writing the second copy of the new environment data.
In this case this first copy contains valid new data, while the second copy of the environment does not contain valid data. In the current implementation, the first (and only) saveenv would have completed, too, and the reset would hit after leaving this part of code, so we had valid new data in the first copy, and valid (but old) data in the second one. Again, this is not an improvement. Actually I think the current implementations is even more useful.Case 3: A flash sector in the first copy of the environment becomes defective while we erase or write it. In this case we will see appropriate error conditions, and the "saveenv" command will abort.
This is the same as case 1: no valid data in copy 1, valid, but old data in copy 2; no difference between the existing and your new implementation.Case 4: A flash sector in the second copy of the environment becomes defective while we erase or write it. In this case we will see appropriate error conditions, and the "saveenv" command will abort.
This is the same as case 2: valid new data in copy 1, no valid data in copy 2 with your implementation, but probably valid old data with the existing code.I guess I must have missed some cases because there was none yet where the new implementaion would improve the reliability. Please fill in these missing cases.
Case 5: Data retention. If you check the data sheets of your flash device, you should find a section on data retention. It probably is more than 10 years Grabbing a Spansion AM29F800B http://www.spansion.com/products/Am29F800B.html data sheet at random, I find it is rated at 10 years at 150^C, 20 years at 125^C.
Even if the retention rating of the part given by the manufacturer is insufficient, a mirror (duplication) of the "redundant" environment that was created at the same time as the "active" environment could not be expected to last any longer than the "active" environment since they were written at the same time.
If (a) you are really paranoid and (b) you expect your gadget to outlive the retention of the memories, you need to rewrite the environment (and, likely, your whole program!) every few years in order to reset the decay. Making a duplicate environment once won't help.
[snip]
So, can you please fill in the szenario where your modification would really help to make the system more reliable?
Best regards, Wolfgang Denk
2cents, and worth every penny you paid, gvb