Re: Preventing JFFS2 partial page writes?

[Artem Bityutskiy <dedekind1@gmail.com>]

On Mon, 2011-06-27 at 10:31 -0400, Peter Barada wrote:
> On 06/24/2011 03:26 PM, Artem Bityutskiy wrote:
> > On Wed, 2011-06-22 at 11:28 -0400, Peter Barada wrote:
> >> Thoughts?
> > Sorry, could you please define the problem you are trying to solve?
> > Sorry if you did define it in your long post, but I could not easily
> > find it.
> The problem I'm trying to solve is that the Micron NAND  I'm using has 
> an internal 4-bit ECC engine and uses four 8-byte ECCs that provide 
> 4-bit protection per 512 data bytes + four OOB bytes.  The ecclayout I'm 
> using is:
> 
> ecclayout = {
>          eccbytes = 32,
>          eccpos = { 8, 9, 10, 11, 12, 13, 14, 15, /* ECC data bytes 
> 0-511 + OOB bytes 4-7 */
>                  24, 25, 26, 27, 28, 19, 30, 31, /* ECC data bytes 
> 512-1023 + OOB bytes 20-23 */
>                  40, 41, 42, 43, 44, 45, 46, 47,  /* ECC data bytes 
> 1024-1535 + OOB bytes 36-39 */
>                  56, 57, 58, 59, 60, 61, 62, 63},  /* ECC data bytes 
> 1536-2047 + OOB bytes 52-55 */
>          .oobfree = {
>                  { .offset = 4,
>                    .length = 4},
>                  { .offset = 20,
>                    .length = 4},
>                  { .offset = 36,
>                    .length = 4},
>                  { .offset = 52,
>                    .length = 4},
>          },
> };
> 
> After the JFFS2 cleanmarker is written into bytes 4-7 and 16-23 of the 
> OOB, nanddump shows:
> 
>    OOB Data: ff ff ff ff 85 19 03 20 5a e3 da 69 01 40 f1 36
>    OOB Data: ff ff ff ff 08 00 00 00 91 99 3c 05 01 d0 5d b3
>    OOB Data: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>    OOB Data: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> 
> Note that the ECC bytes 8-15 and 24-31 are no longer FF due to bytes 4-7 
> and bytes 20-23 being written with non-zero data.
> 
> When data is later written to this same page (w/o an intervening erase 
> of its block) reading the page causes an uncorrectable ECC error.
> 
> There are eight additional bytes of OOB space available for writing, but 
> they are not ECC'd.
> 
> The issue I'm trying to solve is how to communicate from MTD to JFFS2 
> that some bytes of the oobfree array perturb the data ECC and can not be 
> used to write the cleanmarker.

OK, thanks for explanation. I am not very good in this area as I do not
have much experience dealing with OOB, but here is what I thing.

1. Linux MTD code was _not_ designed for "ECC'ed OOB".
2. I do not really know what MTD_OOB_RAW is, and the comment in mtd.h
   is not very verbose.
3. But in my opinion MTD_OOB_AUTO makes most sense and should be used
   everywhere except for some tricky cases when you want to test things
   by writing incorrect ECC, or you have an image with ECC and you want
   to flash it as is.
4. In general, OOB should be considered as belonging to the driver, and
   modern software should not rely on OOB at all.
5. So MTD_OOB_AUTO make free bytes in OOB look like a contiguous buffer
   which the _user_ can freely and _independently_ use.
6. In your case only this assumption does not work and your ecclayout is
   incorrect because the OOB areas you expose are not independent.
7. So in your case your ecclayout should be changed and you should
   expose only independent ECC bytes.

-- 
Best Regards,
Artem Bityutskiy