[v3,1/2] mtd: rawnand: marvell: document a bit more the driver

A stale document about the old pxa3cc_nand.c driver is available in
Documentation/mtd/nand/. Rewrite the parts that explain the IP itself
and some non-trivial choices made in the driver directly in
marvell_nand.c to then be able to remove this file.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---

Changes since v2:

On Sat,  4 Aug 2018 11:57:16 +0200
Miquel Raynal <miquel.raynal@bootlin.com> wrote:

> A stale document about the old pxa3cc_nand.c driver is available in
> Documentation/mtd/nand/. Rewrite the parts that explain the IP itself
> and some non-trivial choices made in the driver directly in
> marvell_nand.c to then be able to remove this file.
> 
> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> ---
> 
> Changes since v2:
> =================
> * Addressed Boris comments, mostly typos and rewritting as suggested.
> 
> Changes since v1:
> =================
> * Corrected mistakes pointed by Boris.
> * Enlarged the documentation as suggested by Thomas to mention the
>   layouts as seen per the controller.
> * Added a mention about the maximum ECC size which is 2kiB.
> 
> 
>  drivers/mtd/nand/raw/marvell_nand.c | 63 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 63 insertions(+)
> 
> diff --git a/drivers/mtd/nand/raw/marvell_nand.c b/drivers/mtd/nand/raw/marvell_nand.c
> index 218e09431d3d..dc1da132e684 100644
> --- a/drivers/mtd/nand/raw/marvell_nand.c
> +++ b/drivers/mtd/nand/raw/marvell_nand.c
> @@ -5,6 +5,69 @@
>   * Copyright (C) 2017 Marvell
>   * Author: Miquel RAYNAL <miquel.raynal@free-electrons.com>
>   *
> + *
> + * This NAND controller driver handles two versions of the hardware,
> + * one is called NFCv1 and is available on PXA SoCs and the other is
> + * called NFCv2 and is available on almost Armada SoCs.

					     ^ all

BTW, do we really have Armada SoCs without a NAND controller IP?

> + *
> + * The main visible difference is that NFCv1 only has Hamming ECC
> + * capabilities, while NFCv2 also embeds a BCH ECC engine. Also, DMA
> + * is not used with NFCv2.
> + *
> + * The ECC layouts are depicted in details in Marvell AN-379.

"
The ECC layouts are depicted in details in Marvell AN-379, but here
is a brief description.
"


> + *
> + * When using Hamming, the data is split in 512B chunks (either 0, 1
> + * or 2)

0 chunks, really?

> and each chunk will have its own ECC "digest" of 6B at the
> + * beginning of the OOB area and eventually the remaining fee OOB

							     ^ free

> + * bytes. This engine corrects up to 1 bit per chunk and detects
> + * reliably an error if there are at most 2 bitflips. Here the page

							      ^ is

> + * layout used by the controller when Hamming is chosen:
> + *
> + * +-------------------------------------------------------------+
> + * | Data 1 | ... | Data N | ECC 1 | ... | ECCN | Free OOB bytes |
> + * +-------------------------------------------------------------+
> + *
> + * When using the BCH engine, there are N identical (data + spare +
> + * ECC) sections and potentially an extra one to deal with
> + * configurations where the chosen (data + spare + ECC) sizes do not align
> + * with the page (data + OOB) size. Here the page layout used by

					   ^ is

> + * the controller when BCH is chosen:
> + *
> + * +-----------------------------------------
> + * | Data 1 | Free OOB bytes 1 | ECC 1 | ...
> + * +-----------------------------------------
> + *
> + *      -------------------------------------------
> + *       ... | Data N | Free OOB bytes N | ECC N |
> + *      -------------------------------------------
> + *
> + *           --------------------------------------------+
> + *            Last Data | Last Free OOB bytes | Last ECC |
> + *           --------------------------------------------+
> + *
> + * In both cases, the layout seen by the user is always: all the
> + * data in one chunk, all the free OOB bytes and finally all the ECC

    ^ all data first, then all free OOB bytes and finally all ECC bytes.

And this is something you already re-explain. Maybe you should just say
it in one place.

> + * bytes. With BCH, ECC bytes are 30B long and are padded to 32B with

							     ^ with 0xff
      to align on 32 bytes.

Also, you should say that it's 30bytes per ECC chunck, not for the
whole page. That's not clearly stated here.

> + * 0xFF.
> + *
> + * The controller has certain limitations that are handled by the
> + * driver:
> + *   - It can only read 2k at a time. To overcome this limitation, the
> + *     driver makes use of 'naked' operations.

'naked' operations is a Marvel term. You should explain briefly what
this is (issue data cycles on the bus without issuing new CMD+ADDR
cycles).

> + *   - The ECC strength in BCH mode cannot be tuned easily. It is

It cannot be tuned at all :P.

> + *     fixed 16 bits. What can be tuned in the ECC block size as long

					   ^ is

> + *     as it stays between 512B and 2kiB. It's usually chosen based on
> + *     the chip ECC requirements. For instance, using 2kiB ECC chunks
> + *     provides 4b/512B correctability.
> + *   - The controller will always treat data bytes, free OOB bytes
> + *     (also referred as "spare bytes") and ECC bytes in that order,

The spare and free OOB bytes term have already been used before, so
it's probably too late to tell that they are the same thing :).

> + *     no matter the real standard layout (which is usually all data
> + *     then all OOB bytes). The marvell_nfc_layouts array below
> + *     contains the currently supported layouts.
> + *   - Because of these weird layouts, the Bad Block Markers can be
> + *     located in data section. In this case, the NAND_BBT_NO_OOB_BBM
> + *     option must be set to prevent scanning/writing bad block
> + *     markers..
>   */
>  
>  #include <linux/module.h>