mtd: nand: add support for Micron on-die ECC

Now that the core NAND subsystem has support for on-die ECC, this commit
brings the necessary code to support on-die ECC on Micron NANDs.

In micron_nand_init(), we detect if the Micron NAND chip supports on-die
ECC mode, by checking a number of conditions:

 - It must be an ONFI NAND
 - It must be a SLC NAND

 - Enabling *and* disabling on-die ECC must work

 - The on-die ECC must be correcting 4 bits per 512 bytes of data. Some
   Micron NAND chips have an on-die ECC able to correct 8 bits per 512
   bytes of data, but they work slightly differently and therefore we
   don't support them in this patch.

Then, if the on-die ECC cannot be disabled (some Micron NAND have on-die
ECC forcefully enabled), we bail out, as we don't support such
NANDs. Indeed, the implementation of raw_read()/raw_write() make the
assumption that on-die ECC can be disabled. Support for Micron NANDs
with on-die ECC forcefully enabled can easily be added, but in the
absence of such HW for testing, we preferred to simply bail out.

If the on-die ECC is supported, and requested in the Device Tree, then
it is indeed enabled, by using custom implementations of the
->read_page(), ->read_page_raw(), ->write_page() and ->write_page_raw()
operation to properly handle the on-die ECC.

In the non-raw functions, we need to enable the internal ECC engine
before issuing the NAND_CMD_READ0 or NAND_CMD_SEQIN commands, which is
why we set the NAND_ECC_CUSTOM_PAGE_ACCESS option at initialization
time (it asks the NAND core to let the NAND driver issue those
commands).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>

include/linux/mtd/nand.h

@@ -258,6 +258,8 @@ struct nand_chip;
 
 /* Vendor-specific feature address (Micron) */
 #define ONFI_FEATURE_ADDR_READ_RETRY	0x89
+#define ONFI_FEATURE_ON_DIE_ECC		0x90
+#define   ONFI_FEATURE_ON_DIE_ECC_EN	BIT(3)
 
 /* ONFI subfeature parameters length */
 #define ONFI_SUBFEATURE_PARAM_LEN	4