MTD NAND BCH support for 24 bits/1K of ECC correction?

Hi all,

I am trying to clean up our OCTEON NAND flash driver in the Linux kernel and enable support for multi-bit ECC using BCH and am having some issues. I am able to successfully work with NAND flash that requires 4 bits ECC per 512 bytes but I am having issues with one of our boards that has a NAND device that requires 24 bits of ECC per 1024 bytes.

I was wondering if ECC of this magnitude has been successfully tested in the past. By my calculations I should have 42 bytes of ECC per 1K block (m=14, t=24 for 336 bits of ECC data). My problem is that when decoding an encoded block I am seeing that nroots != err in decode_bch() after find_poly_roots(). I am seeing this for all of the blocks I attempt to read. As far as I can tell the data being sent to BCH is good, though it might have a few bad bits but nowhere near 24.

I am also seeing this same behavior in my U-Boot code which uses the identical bch and nand_bch code.

Quoted 16 lines · Aaron Williams

Hi all,

I am trying to clean up our OCTEON NAND flash driver in the Linux kernel and enable support for multi-bit ECC using BCH and am having some issues. I am able to successfully work with NAND flash that requires 4 bits ECC per 512 bytes but I am having issues with one of our boards that has a NAND device that requires 24 bits of ECC per 1024 bytes.

I was wondering if ECC of this magnitude has been successfully tested in the past. By my calculations I should have 42 bytes of ECC per 1K block (m=14, t=24 for 336 bits of ECC data). My problem is that when decoding an encoded block I am seeing that nroots != err in decode_bch() after find_poly_roots(). I am seeing this for all of the blocks I attempt to read. As far as I can tell the data being sent to BCH is good, though it might have a few bad bits but nowhere near 24.

Quoted 3 lines

I am also seeing this same behavior in my U-Boot code which uses the identical bch and nand_bch code.

Hi Aaron,

CC-ing your message to linux-mtd which is the place to go for such questions :-)

Your configuration (m=14 t=24 with 1024 bytes of data) has been tested, and should work with the BCH library. Could you give some details about your ECC setup:

1. Are you trying to locate and correct errors from hardware-computed syndromes ?

2. If yes, did you provide the BCH lib with the specific primitive polynomial used by your hardware ? What is this polynomial ?

3. Could you provide the ECC bytes generated for the following block patterns:
- a 0xff-filled 1024 bytes block
- a 0xff-filled 1024 bytes block, except for the first byte set to 0xfe This would help me find out how to setup the library to match your hardware.

Regards,