additional error checks for AG-AND erase/write

[David A. Marlin]

Thomas Gleixner wrote:
	:
> 
> Rename nand_read_ecc to do_nand_read_ecc and add an flag argument. Look
> at the erase function, where we have the additional argument for
> allowbbt. 
> flag bits:	0-7 tolerable errors
> 		8   do not get chip
> or something like that.
> Create a new wrapper function nand_read_ecc which calls do_nand_read_ecc
> with flags = 0xff. Depending on this argument we grab the device and
> release it on the end and react on the error check.
> The same call can be done from erase in the failure path. 
> 
> To make this actually work we should add a member like
> int (*ext_errchk) (.....)
> which is checked in the error path
> if (this->ext_errchk) {
> 	res = this->ext_errchk(....);
> }
> This is a board supplied function, as the board driver knows how much
> errors are acceptable. We return the number of corrected bits anyway in
> the correct_data() function, but we check only for -1 at the moment.
> An additional check does not hurt here, as we are in the slow path
> again. We check the return value of correct_data() for 
> (res == -1 || res > flags & 0xff) which ensures in the normal case this
> tolerance will never bite us, as error correction of 256 bits is unreal.
> 
> Hope that helps. I think this will make the code not uglier than it is
> anyway and keeps it nicely dependend on the board drivers featurelist.


Attached is a patch that I think implements what you described.  Please 
let me know if I've missed anything or gone astray in the details.

Note:  I changed a few literals to defined symbols in 'nand_base.c'. 
Please let me know if you would prefer this in a separate patch (or not 
at all).


Thank you,

d.marlin
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