mtd: nand: provide detailed description for raw read/write page methods

read_page_raw and write_page_raw method description is not clear enough.
It clearly specifies that ECC correction should not be involved but does
not talk about specific layout (by layout I mean where in-band and
out-of-band data are stored on the NAND media) used by NAND/ECC
controllers.

Those specific layouts might impact MTD users and thus should be hidden (as
already done in the standard NAND_ECC_HW_SYNDROME implementation).

Clearly state this constraint in the nand_ecc_ctrl struct documentation.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

include/linux/mtd/nand.h

@@ -455,8 +455,21 @@ struct nand_hw_control {
  *		be provided if an hardware ECC is available
  * @calculate:	function for ECC calculation or readback from ECC hardware
  * @correct:	function for ECC correction, matching to ECC generator (sw/hw)
- * @read_page_raw:	function to read a raw page without ECC
- * @write_page_raw:	function to write a raw page without ECC
+ * @read_page_raw:	function to read a raw page without ECC. This function
+ *			should hide the specific layout used by the ECC
+ *			controller and always return contiguous in-band and
+ *			out-of-band data even if they're not stored
+ *			contiguously on the NAND chip (e.g.
+ *			NAND_ECC_HW_SYNDROME interleaves in-band and
+ *			out-of-band data).
+ * @write_page_raw:	function to write a raw page without ECC. This function
+ *			should hide the specific layout used by the ECC
+ *			controller and consider the passed data as contiguous
+ *			in-band and out-of-band data. ECC controller is
+ *			responsible for doing the appropriate transformations
+ *			to adapt to its specific layout (e.g.
+ *			NAND_ECC_HW_SYNDROME interleaves in-band and
+ *			out-of-band data).
  * @read_page:	function to read a page according to the ECC generator
  *		requirements; returns maximum number of bitflips corrected in
  *		any single ECC step, 0 if bitflips uncorrectable, -EIO hw error