[PATCH] Towards DiskOnChip support as a NAND driver

[Thomas Gleixner]

On Wednesday 16 June 2004 22:23, Dan Brown wrote:
> > ECC - hardware error correction by the device?  Is this an optional
> > enable in the driver?
>
> Yes, the DOC has an ASIC that does ECC computation (for actual error
> correction, software assitance is required).  It's not optional, though it
> could be made so....  Why would you want to turn it off?

It has two advantages over software ECC.

1. The software ECC is a Hamming code which can correct 1 bit errors and 
detect 2 bit errors. The HW ECC on DOC devices is a Reed Solomon Encoder / 
Decoder which allows to correct and detect more errors

2. In general a HW based ECC generator is a lot faster than the software 
version as it gets you rid of a ugly loop through all the data bytes on each 
read / write.

A small drawback is the error correction for reed solomon, as it is more  
complex and slower than for hamming codes. This is not too bad as error 
correction usually starts to happen when the device wears out, but then the 
hamming code which we use in soft ecc can be rather unsufficient. 
The speed improvement on write and read is really worth to use hw ecc if its 
thers

-- 
Thomas
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