[U-Boot-Users] RFC on davinci Nand support changes

[ksi at koi8.net]

On Wed, 26 Sep 2007, Troy Kisky wrote:

>>
>> Again, you are trying to reinvent the wheel. The ECC algorithm is
> described
>> in details in TI documentation and MontaVista implemented it verbatim.
>>
>>
>> This code is a verbatim implementation of what is described in TI
>> documentation. It's not supposed to look nice, it's supposed to work.
>
> I guess that's what happens when you implement an algorithm without
> understanding it.
> Example: You have a block of all zeros.
>
> The ecc stored in the spare bytes of this is also 0.
> Now, upon reading this block of zeroes, a two bit ecc occurs. The bits
> that happen to be
> read incorrectly are bit # 0 & bit # 0x3f of the block
> The hardware calculated ecc will be
> 0:0 ^ 0:fff = 0:fff after bit 0
> 0:fff ^ 3f:fc00 = 3f:3f after bit 3f
>
> Now, when your algorithm counts bits you get 12, and decide
> it is a single bit ecc error.
>
> I however xor the high and low 12 bits 3f ^ 3f = 0, 0 != fff and
> decide it is multi bit ecc error and give an error.
>
> Note, that both approaches would have decide it was a single bit error,
> if the second
> error wouldn't have happened.
>
> I could give an example of another error with your algorithm, but I have
> no desire to teach those without a desire to learn and no one else on
> this
> list seems interested in this subject.

Eh, that sounds like myself years, say, 20..25 ago...

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