RE: [LINUX PATCH v12 3/3] mtd: rawnand: arasan: Add support for Arasan NAND Flash Controller

[Naga Sureshkumar Relli <nagasure@xilinx.com>]

Hi Miquel,

> -----Original Message-----
> From: Miquel Raynal [mailto:miquel.raynal@bootlin.com]
> Sent: Wednesday, December 19, 2018 7:57 PM
> To: Naga Sureshkumar Relli <nagasure@xilinx.com>
> Cc: Boris Brezillon <boris.brezillon@bootlin.com>; robh@kernel.org; richard@nod.at;
> linux-kernel@vger.kernel.org; marek.vasut@gmail.com; linux-mtd@lists.infradead.org;
> nagasuresh12@gmail.com; Michal Simek <michals@xilinx.com>;
> computersforpeace@gmail.com; dwmw2@infradead.org; martin.lund@keep-it-simple.com
> Subject: Re: [LINUX PATCH v12 3/3] mtd: rawnand: arasan: Add support for Arasan
> NAND Flash Controller
> 
> Hi Naga,
> 
> + Martin
> 
> Naga Sureshkumar Relli <nagasure@xilinx.com> wrote on Tue, 18 Dec 2018
> 05:33:53 +0000:
> 
> > Hi Miquel,
> >
> > > -----Original Message-----
> > > From: Miquel Raynal [mailto:miquel.raynal@bootlin.com]
> > > Sent: Monday, December 17, 2018 10:11 PM
> > > To: Naga Sureshkumar Relli <nagasure@xilinx.com>
> > > Cc: Boris Brezillon <boris.brezillon@bootlin.com>; robh@kernel.org;
> > > richard@nod.at; linux- kernel@vger.kernel.org;
> > > marek.vasut@gmail.com; linux-mtd@lists.infradead.org;
> > > nagasuresh12@gmail.com; Michal Simek <michals@xilinx.com>;
> > > computersforpeace@gmail.com; dwmw2@infradead.org
> > > Subject: Re: [LINUX PATCH v12 3/3] mtd: rawnand: arasan: Add support
> > > for Arasan NAND Flash Controller
> > >
> > > Hi Naga,
> > >
> > > [...]
> > >
> > > > Inserted biterror @ 48/7
> > > > Successfully corrected 25 bit errors per subpage Inserted biterror
> > > > @
> > > > 50/7 ECC failure, invalid data despite read success
> > > > root@xilinx-zc1751-dc2-2018_1:~#
> > > >
> > > > But even in this case also, driver is saying ECC failure but read success.
> > > > That means controller is able to detect errors on read page up to 24 bit only.
> > > > After that there is no way to say to the upper layers that the
> > > > page is bad because of the
> > > limitation in the controller.
> > >
> > > This is more than a "limitation", the design is broken. I am not
> > > sure how to support such controller, and I am not sure if we even want to.
> >
> > The number of errors that are correctable is limited by a parameter
> > 't'(total number of errors), If there is a condition that the number of errors greater than 't',
> then the controller won't be able to detect that.
> > I guess this concept is same for other controllers as well.
> > In Arasan it is limited to 24-bit.
> >
> > Even, in case of Hamming, it is 1-bit error correction and 2-bit error detection.
> > What will happen if there are multiple errors(greater than 2-bit)?
> 
> Ok let's use the Hamming comparison in your ECC engine case.
> 
> -> hamming:
>  * 0 bf: everything is fine
>  * 1 bf: will be detected, corrected, signaled
>  * 2 bf: will be detected, not corrected, signaled
>  * 3+ bf: don't care
> 
> -> BCH:
>  * 0 bf: everything is fine
>  * 1-24 bf: will be detected, corrected, signaled
>  * 25 bf: everything is fine
>  * 26+ bf: don't care
> 
> Do you see the problem?
No.
> 
> In the 25 bf case, the controller is reporting that everything went fine while it should report
> that it detected an uncorrectable situation.
> 
> Here are two leads to solve this issue, please investigate them both:
> 1/ Talk to your colleagues that developed the RTL, ask if there is a
>    hidden/reserved bit for that purpose that is not documented.
I spoke to RTL guys, there is nothing hidden/reserved bit for this purpose.
I tried reading the status registers reserved bits, but they are raz(read as zero)

> 2/ Search for a status in the registers that might indicate that an
>    error occurred, for instance "0 bf corrected" and "bf have been
>    detected".
I tried reading status registers and other registers as well, but no luck.
> 
> NB: I know that, with a BCH ECC engine, error detection at (strength +
> 1) is not 100% sure but statistically it will almost always be detected and in this case we
> need the controller to warn the user!
Ok, I understood now.

Thanks,
Naga Sureshkumar Relli
> 
> 
> Thanks,
> Miquèl