[PATCH v5 1/3] mtd: nand: gpio: Add DT property to automatically determine bus width

[Gupta, Pekon]

Hi Alexander,

> From:  Alexander Shiyan
> This patch adds a property to automatically determine the NAND
> bus width by CFI/ONFI information from chip. This property works
> if the bus width is not specified explicitly.
> 
> Signed-off-by: Alexander Shiyan <shc_work at mail.ru>
> ---
[...]

> --- a/Documentation/devicetree/bindings/mtd/gpio-control-nand.txt
> +++ b/Documentation/devicetree/bindings/mtd/gpio-control-nand.txt
> @@ -19,6 +19,9 @@ Optional properties:
>    defaults to 1 byte.
>  - chip-delay : chip dependent delay for transferring data from array to
>    read registers (tR).  If not present then a default of 20us is used.
> +- gpio-control-nand,bank-width-auto : Device bus width is determined
> +  automatically by CFI/ONFI information from chip if "bank-width"
> +  parameter is omitted (Boolean).
> 
There is one important thing to understand here is that when a READ_ID
or READ_PARAM command is issued to NAND device, the read_data is
is transmitted only on lower 8-bit data lines. This is as per ONFI standard.
Therefore, irrespective of whether the NAND device is x8 or x16,
ONFI parameter page is always read in 8-bit mode.. 
--------------------------
[*] Reference: ONFI spec version 3.1 (section 3.5.3. Target Initialization)
    "The Read ID and Read Parameter Page commands only use the lower 8-bits
     of the data bus. The host shall not issue commands that use a word
     data width on x16 devices until the host determines the device supports
     a 16-bit data bus width in the parameter page."
--------------------------

Now this forms the basis of NAND_BUSWIDTH_AUTO, which allows
ONFI parameter parsing '@@nand_flash_detect_onfi()' only in 8-bit
mode. And once the actual width is known, then it re-configures the
driver while returning from nand_scan_ident().

Thus, in summary: To read on-chip ONFI parameters, your driver
should always be in 8-bit mode.
Using this as basis, we tried eliminate NAND_BUSWIDTH_AUTO and
instead implemented this requirement directly as part of generic
NAND driver in nand_base.c: nand_flash_detect_onfi().

Please refer following patch and subsequent discussion on same.
(It might help you understand that we don't need to explicitly
set have NAND_BUSWIDTH_AUTO, instead we can make it implicit
in our code while scanning NAND devices)

http://lists.infradead.org/pipermail/linux-mtd/2013-November/050252.html


with regards, pekon