Re: Hardware ECC in NAND flash driver

[Thomas Gleixner <tglx@linutronix.de>]

On Sat, 2002-08-10 at 12:33, Thomas Gleixner wrote:
> More comments later today.
Hi Steven,

back again. I tried several possibilities to deal with all aspects. I
picked up most of your ideas and worked them into the existing code. My
final conclusion is as follows: 

struct nand_chip got following extensions:

calculate_ecc - function for ecc calculation or readback from ecc
hardware, defaults to nand_calculate_ecc for software ecc
correct_data - function for ecc correction, matching to ecc generator
(sw/hw), defaults to nand_correct_data for software ecc
enable_hwecc - function to enable (reset) hardware ecc generator
eccmod - mode of ecc: see constants
eccsize - databytes used per ecc-calculation, set by nand.c depending on
eccmod value

following eccmodes are defined at the moment

#define NAND_ECC_NONE		0 
is forced, if CONFIG_MTD_NAND_ECC is not set
#define NAND_ECC_SOFT		1 
is forced, if CONFIG_MTD_NAND_ECC is set and no hardware ecc is selected
#define NAND_ECC_HW3_256	2 
user selectable, if your ecc hardware supports 3 byte ecc for 256 byte
of data (DOC or passive SmartMedia adaptors)
#define NAND_ECC_HW3_512	3
user selectable, if your ecc hardware supports 3 byte ecc for 512 byte
of data (Samsung S3C2410)

If you select hardware ecc in your driver, you have to take care, that
calculate_ecc, correct_data and enable_hwecc are filled with the
relevant function pointers and eccmod is set to the appropriate type,
before you call nand_scan.

I'm still not willing to write already written data to the device for a
second time. So I decided to skip hardware ecc for that case. If you use
a NAND aware filesystem e.g. JFFS2 this should not happen, otherwise you
will get in trouble anyway. 

The funtion, which does correct_data calculations for the specfic
hardware ecc should be implemented as a general function in nand_ecc.c
to make it usable for similar hardware drivers and to avoid duplicated
code in these drivers.

Please remove the ECC reset out of your command function and move it to
a seperate function, which will be called by enable_hwecc.

Get code from CVS and enjoy. Thanks for your suggestions. The standard
software ecc mode runs on my machine without complains. I tested HW3_256
mode with software ecc routines and a dummy enable_hwecc and found no
problems. Any suggestions and improvements are welcome. 
Happy testing.

-- 
Thomas 
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