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Date:   Mon, 22 Mar 2021 19:32:13 +0100
From:   Miquel Raynal <miquel.raynal@bootlin.com>
To:     Daniel Palmer <daniel@0x0f.com>
Cc:     linux-mtd@lists.infradead.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] mtd: spinand: add support for Foresee FS35ND01G-S1Y2
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Hi Daniel,

Daniel Palmer <daniel@0x0f.com> wrote on Mon, 22 Mar 2021 21:44:40
+0900:

> Hi Miquel,
> 
> Sorry for the resend. Gmail randomly switched to HTML email so the
> original version seems to have bounced.
> 
> On Mon, 15 Feb 2021 at 20:16, Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> > > "2. Spare area 800H to 83FH is all available for user.
> > >  ECC parity codes are programmed in
> > > additional space and not user accessible."
> > >
> > > It would seem that the pages are actually bigger than 2K + 64 or there
> > > is some other place they keep the ECC.
> > > Or both datasheets are lying. Somewhere else in the datasheets it says
> > > that writes to the ECC area will be ignored but that doesn't make a
> > > lot of sense if the ECC area isn't user accessible in the first place.
> > >
> > > I didn't think about it at the time but I can take a dump of the OOB
> > > area of my FS35ND01G-S1Y2 to confirm it's all 0xff except for any
> > > factory marked bad blocks.  
> >
> > I see. Can you please try the following:
> >
> > nandwrite -o /dev/mtdx /dev/zero
> > nanddump -ol1 /dev/mtdx
> > If the entire area is effectively free to be used, you should see 0's
> > everywhere. Otherwise you should have ff's somewhere.  
> 
> Sorry I didn't follow up sooner on this. I needed to order another of
> this flash chip to test with as I couldn't destroy the data on the one
> I have.
> 
> Anyhow:
> 
> Erased the page with flash erase (I'm forcing it to erase bad blocks
> here as I mess up the marker, I have a hack to allow erasing bad
> blocks..)
> Everything is 0xFF for that page.
> 
> # flash_erase -N /dev/mtd1 0 1
> Erasing 128 Kibyte @ 0 -- 100 % complete
> # nanddump --bb=dumpbad -n -l2048 -o -c -s 0x0 /dev/mtd1
> Block size 131072, page size 2048, OOB size 64
> Dumping data starting at 0x00000000 and ending at 0x00000800...
> 0x00000000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  |................|
> ....
> 0x000007f0: 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  |................|
>   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  |................|
>   OOB Data: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  |................|
> 
> Write zeros into the page and OOB.
> Get all zeros back including the OOB.
> 
> # nandwrite -o /dev/mtd1 /dev/zero
> Writing data to block 0 at offset 0x0
> Bad block at 0, 1 block(s) will be skipped
> Writing data to block 1 at offset 0x20000
> # nandwrite -N -o /dev/mtd1 /dev/zero
> Writing data to block 0 at offset 0x0
> # nanddump --bb=dumpbad -n -l2048 -o -c -s 0x0 /dev/mtd1
> Block size 131072, page size 2048, OOB size 64
> Dumping data starting at 0x00000000 and ending at 0x00000800...
> 0x00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  |................|
> ...
> 0x000007f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  |................|
>   OOB Data: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  |................|
>   OOB Data: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  |................|
>   OOB Data: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  |................|
>   OOB Data: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> |................|
> 
> Erase the page again and writing random junk into it.
> Seeing random junk everywhere including the OOB.
> 
> # flash_erase -N /dev/mtd1 0 1
> Erasing 128 Kibyte @ 0 -- 100 % complete
> # nandwrite -N -o /dev/mtd1 /dev/urandom
> Writing data to [  230.506260] random: nandwrite: uninitialized
> urandom read (2048 bytes read)
> block 0 at offse[  230.514705] random: nandwrite: uninitialized
> urandom read (64 bytes read)
> t 0x0
> # nanddump --bb=dumpbad -n -l2048 -o -c -s 0x0 /dev/mtd1
> Block size 131072, page size 2048, OOB size 64
> Dumping data starting at 0x00000000 and ending at 0x00000800...
> 0x00000000: 5e 24 bd 5f d9 c6 ce c5 b1 85 52 4d 27 94 c9 98  |^$._......RM'...|
> ...
> 0x000007f0: fa 9f 7f 7d ce 99 33 88 d6 9f 99 7d 84 e7 0c 4d  |...}..3....}...M|
>   OOB Data: b1 81 07 6a 8d 47 8b ed 89 88 ac 62 e8 ae 48 54  |...j.G.....b..HT|
>   OOB Data: 7d b2 ea 73 f3 29 ba 65 e6 45 cb 8b 1a c6 5b dc  |}..s.).e.E....[.|
>   OOB Data: b2 2e 77 56 e0 e1 04 59 86 31 7a e5 bd 43 f9 48  |..wV...Y.1z..C.H|
>   OOB Data: 52 05 b2 f1 65 64 59 22 79 50 ec 89 55 6b 6e 23  |R...edY"yP..Ukn#|
> 
> I think this shows that the datasheet is right in that the complete 64
> bytes of "spare area" is usable.
> I have no idea where it puts the ECC though. :)

Argh, I don't like when hardware tries to be smart.

Ok then let's declare no ECC bytes in the OOB layout, I guess it's the
best thing to do...

Thanks for checking btw!

I don't recall the state of the patch which triggered this discussion,
so I guess it's a good time to respin.

Cheers,
Miquèl