Re: [PATCH v3 4/4] mtd: nand: omap: Documentation: How to select correct ECC scheme for your device ?

Re: [PATCH v3 4/4] mtd: nand: omap: Documentation: How to select correct ECC scheme for your device ?

[Brian Norris]

+ devicetree@vger.kernel.org

On Fri, May 09, 2014 at 02:29:15PM +0530, Pekon Gupta wrote:
>  - Adds DT binding property for BCH16 ECC scheme
>  - Adds describes on factors which determine choice of ECC scheme for particular device
> 
> Signed-off-by: Pekon Gupta <pekon@ti.com>
> ---
>  .../devicetree/bindings/mtd/gpmc-nand.txt          | 39 ++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/mtd/gpmc-nand.txt b/Documentation/devicetree/bindings/mtd/gpmc-nand.txt
> index 5e1f31b..f2dbb33 100644
> --- a/Documentation/devicetree/bindings/mtd/gpmc-nand.txt
> +++ b/Documentation/devicetree/bindings/mtd/gpmc-nand.txt
> @@ -28,6 +28,8 @@ Optional properties:
>  		"ham1"		1-bit Hamming ecc code
>  		"bch4"		4-bit BCH ecc code
>  		"bch8"		8-bit BCH ecc code
> +		"bch16"		16-bit BCH ECC code
> +		Refer below "How to select correct ECC scheme for your device ?"
>  
>   - ti,nand-xfer-type:		A string setting the data transfer type. One of:
>  
> @@ -90,3 +92,40 @@ Example for an AM33xx board:
>  		};
>  	};
>  
> +How to select correct ECC scheme for your device ?
> +--------------------------------------------------
> +Higher ECC scheme usually means better protection against bit-flips and
> +increased system lifetime. However, selection of ECC scheme is dependent
> +on various other factors like;
> +(1) Presence of supporting hardware engines on SoC.
> +	Some legacy OMAP SoC do not have ELM h/w engine thus such SoC cannot
> +	support BCHx_HW ECC schemes. But such SoC can support
> +	BCHx_HW_DETECTION_SW ECC schemes which use s/w library with slight
> +	CPU performance panalty only when too bit-flips are detected.

s/panalty/penalty/

Please do not refer to Linux Kconfig symbols and jargon. It's already
confusing to understand what your various permutations of
BCHx_HW_DETECTION_SW mean. Please simply speak of hardware support and
software library support that can be used to fill in the gaps, using
plain English. This is not the place to discuss Linux details.

> +(2) Device parameters like OOBSIZE
> +	Higher ECC schemes require more OOB/Spare area to store ECC.
> +	So choice of ECC scheme is limited by NAND oobsize. In general
> +	following expression help determine whether given device can
> +	accomodate ECC syndrome or not:
> +	"2 + (PAGESIZE / 512) * ECC_BYTES" >= OOBSIZE
> +	where
> +		OOBSIZE		number of bytes in OOB/spare area
> +		PAGESIZE	number of bytes in main-area of device page
> +		ECC_BYTES	number of ECC bytes generated to protect
> +		                512 bytes of data, which is:
> +				'3' for HAM1_xx ecc schemes
> +				'7' for BCH4_xx ecc schemes
> +				'14' for BCH8_xx ecc schemes
> +				'26' for BCH16_xx ecc schemes
> +
> +	Example(a): For a device with PAGESIZE = 2048 and OOBSIZE = 64
> +		Number of spare/OOB bytes required for using BCH16 ecc-scheme
> +		"(2 + (2048 / 512) * 26) = 106 bytes" is greater than OOBSIZE
> +		(As per above table for BCH16 ecc-scheme, ECC_BYTES = 26)
> +		Thus BCH16 cannot be supported on 2K NAND with OOBSIZE=64 bytes
> +
> +	Example(b): For a device with PAGESIZE = 2048 and OOBSIZE = 128
> +		Number of spare/OOB bytes required for using BCH16 ecc-scheme
> +		"(2 + (2048 / 512) * 26) = 106 bytes" is less than OOBSIZE
> +		(As per above table for BCH16 ecc-scheme, ECC_BYTES = 26)
> +		Thus BCH16 can be supported on 4K NAND with OOBSIZE=128 bytes

Brian