Re: [PATCH 1/3] of: mtd: add helper reading "nand-ecc-algo" from DT

[Boris Brezillon <boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

On Thu, 25 Feb 2016 20:56:36 +0100
Rafał Miłecki <zajec5-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> On 24 February 2016 at 14:46, Boris Brezillon
> <boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org> wrote:
> > On Fri, 12 Feb 2016 19:11:23 +0100
> > Rafał Miłecki <zajec5-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >
> >> This allows specifying algorithm used for NAND ECC. There are two
> >> available values: "bch" and "hamming". It's important as having just
> >> ECC parameters (step, strength) isn't enough to determine algorithm,
> >> e.g. you can't distinct BCH-1 and Hamming.
> >>
> >> Signed-off-by: Rafał Miłecki <zajec5-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> >> ---
> >>  Documentation/devicetree/bindings/mtd/nand.txt |  3 +++
> >>  drivers/of/of_mtd.c                            | 33 ++++++++++++++++++++++++++
> >>  include/linux/mtd/nand.h                       |  5 ++++
> >>  include/linux/of_mtd.h                         |  6 +++++
> >>  4 files changed, 47 insertions(+)
> >>
> >> diff --git a/Documentation/devicetree/bindings/mtd/nand.txt b/Documentation/devicetree/bindings/mtd/nand.txt
> >> index b53f92e..a2c2df5 100644
> >> --- a/Documentation/devicetree/bindings/mtd/nand.txt
> >> +++ b/Documentation/devicetree/bindings/mtd/nand.txt
> >> @@ -3,6 +3,9 @@
> >>  - nand-ecc-mode : String, operation mode of the NAND ecc mode.
> >>    Supported values are: "none", "soft", "hw", "hw_syndrome", "hw_oob_first",
> >>    "soft_bch".
> >> +- nand-ecc-algo : string, algorithm of NAND ecc.
> >> +               Supported values are: "bch", "hamming". The default one is
> >> +               "bch".
> >
> > I like the idea of specifying which ECC algorithm should be used, and
> > this is IMO clearer than putting the information directly in
> > nand-ecc-mode (as is already done for the "soft" and "soft_bch" modes,
> > which are respectively encoding software hamming and software bch
> > implementations).
> >
> > But shouldn't we take this even further and add new DT properties
> > to encode the ECC layout (syndrome, oob_first), and another one to
> > specify the implementation type (software or hardware)?
> >
> > nand-ecc-layout = "nornal", "syndrome" or "oob_first"
> > nand-ecc-engine = "none", "soft" or "hw" ("on-die" ?)
> >
> > Note that it's not something I ask you to do right now, I just want to
> > bring the topic on the table.
> 
> So far I was fine with whatever drvier/NAND core picked, didn't have
> any issue with it. If at some point we'll see a real need of
> specifying it manually as well, I guess we should do as you suggested.
>

My point is that it's kind of weird to have the same information
duplicated in two different properties with possibly some conflicting
configurations, like:

	nand-ecc-mode = "soft_bch"; /* software BCH implementation... */
	nand-ecc-algo = "hamming"; /* ... and here you choose Hamming */

Of course this won't be a real problem until NAND core starts using
this property to decide which soft implementation should be used, but
providing a consistent binding is important IMO.

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