Re: Testing generic empty page bit flips recovery

["Franklin S Cooper Jr." <fcooper@ti.com>]

On 12/30/2015 10:52 AM, Steve deRosier wrote:
> On Wed, Dec 30, 2015 at 8:40 AM, Franklin S Cooper Jr. <fcooper@ti.com> wrote:
>> One thing that confuses me is if I repeatedly call nanddump
>> I continue to get the "ECC: 4 corrected bitflips" message
>> and the "ECC corrected" count increases by 4 each time. If
>> these bits are being corrected which is apparent from
>> looking at the output of nanddump shouldn't sequential calls
>> indicate that no bitflips needed to be corrected since it
>> was corrected previously?
>>
> Hi Franklin,
>
> I'm making a guess at the source of your confusion, but I've had to
> answer this to repeated colleagues recently, so I'll give it a try.
>
> I think you're expecting the "correction" to actually be written and
> fixed on-flash. This is not the case.
>
> Bitflip corrections are applied as the flash data is read. Every time.
> So once a bit is flipped on the NAND, it will always stay that way on
> the physical device until erased and rewritten. Which doesn't happen
> for something as minor as a single bit-flip. So from that point on, it
> will always read and correct the flip and report it to the kernel log.
>
> UBI will move the data once the threshold gets hit, but up until that
> point it will continue to read the same bitflip and correct it.
>
> Hope that helps.
>
> - Steve

Steve,
That makes perfect sense. Thanks for explaining.