linux-kernel - Re: [PATCH] mtd: nand: gpmi: add proper raw access support

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Date:	Mon, 15 Sep 2014 22:12:10 +0200
From:	Boris BREZILLON <boris.brezillon@...e-electrons.com>
To:	Huang Shijie <shijie8@...il.com>
Cc:	Brian Norris <computersforpeace@...il.com>,
	Huang Shijie <shijie.huang@...el.com>,
	Mike Voytovich <mvoytovich@...pal.com>,
	linux-kernel@...r.kernel.org, Huang Shijie <b32955@...escale.com>,
	linux-mtd@...ts.infradead.org, Roy Lee <roylee@...pal.com>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] mtd: nand: gpmi: add proper raw access support

On Mon, 15 Sep 2014 22:43:02 +0800
Huang Shijie <shijie8@...il.com> wrote:

> On Sat, Sep 13, 2014 at 10:38:41AM -0700, Brian Norris wrote:
> > On Sat, Sep 13, 2014 at 11:36:24PM +0800, Huang Shijie wrote:
> > > On Fri, Sep 12, 2014 at 02:30:50PM +0200, Boris BREZILLON wrote:
> > > > This test validates what's returned by ecc_strength file in sysfs
> > > > (which in turn is specified by the NAND controller when initializing
> > > > the NAND chip).
> > > > 
> > > > Doing this should not imply knowing the ECC algorithm in use in the
> > > > NAND controller or the layout used to store data on NAND.
> > > the difficulty is that the ECC parity area can be not byte aligned.
> > 
> > Is there a problem with just rounding up to the nearest byte alignment
> > and ignoring the few bits that are wasted?
> 
> I feel a little confused with the two hooks.
> 
> does the ecc->write_page_raw need to write the ECC parity data?

Depending on the oob_required argument, it might be allowed to
overwrite the ECC bytes even if this implies breaking page reliability
(which is exactly what's expected).

When using raw write with with oob write option the writer should take
care of regenerating ECC bytes (which you said was impossible in GPMI
case) or copying them from a previous raw read.

Here is a real example of what one could test with raw write + oob:
1) read a page in raw mode
2) flip some bits in the generated ECC bytes (or what you references as
   parity data) (this case can actually happen in real life)
3) write the modified page in raw mode
4) read back the same page in normal and check that ECC correction still
   works as expected

> 
> The page's layout after an ecc->write_page  will look like this:
>                                               (Block Mark)
>                                                     |      |
>                                                     |  D   |
>                                                     |<---->|
>                                                     V      V
>      +---+----------+-+----------+-+----------+-+----------+-+
>      | M |   data   |E|   data   |E|   data   |E|   data   |E|
>      +---+----------+-+----------+-+----------+-+----------+-+
> 
> What will the page's layout look like after the ecc->write_page_raw?

The same, but the user won't see this specific layout when reading a
page in raw mode, and won't have to bother about disposing data a
described above when writing in raw mode.

Here is the layout as seen by an MTD user, even though the real one is
the one you previously described.

                                             (Block Mark)
                                                  |
                                                  |
                                                  |
                                                  V
      +------------------------------------------+-----+-------+---+
      |                     data                 |  M  |   E   | R |
      +------------------------------------------+-----+-------+---+

                                                  <------OOB------->


Best Regards,

Boris


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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