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>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>

drivers/mtd/nand/raw/marvell_nand.c

@@ -5,6 +5,73 @@
  * 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 Armada SoCs.
+ *
+ * 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, but here
+ * is a brief description.
+ *
+ * When using Hamming, the data is split in 512B chunks (either 1, 2
+ * or 4) and each chunk will have its own ECC "digest" of 6B at the
+ * beginning of the OOB area and eventually the remaining free OOB
+ * bytes (also called "spare" bytes in the driver). This engine
+ * corrects up to 1 bit per chunk and detects reliably an error if
+ * there are at most 2 bitflips. Here is the page 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 + free OOB +
+ * ECC) sections and potentially an extra one to deal with
+ * configurations where the chosen (data + free OOB + ECC) sizes do
+ * not align with the page (data + OOB) size. ECC bytes are always
+ * 30B per ECC chunk. Here is the page layout used by 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 data
+ * first, then all free OOB bytes and finally all ECC bytes. With BCH,
+ * ECC bytes are 30B long and are padded with 0xFF to align on 32
+ * bytes.
+ *
+ * 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 issues data cycles on the bus, without issuing new
+ *     CMD + ADDR cycles. The Marvell term is "naked" operations.
+ *   - The ECC strength in BCH mode cannot be tuned. It is fixed 16
+ *     bits. What can be tuned is the ECC block size as long 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
+ *     and ECC bytes in that order, no matter what the real layout is
+ *     (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>