Re: [PATCH v7] mtd: rawnand: use bit-wise majority to recover the contents of ONFI parameter

[Chris Moore <moore@free.fr>]

Hi,

Le 13/05/2018 à 06:30, Wan, Jane (Nokia - US/Sunnyvale) a écrit :
> Per ONFI specification (Rev. 4.0), if all parameter pages have invalid CRC values, the bit-wise majority may be used to recover the contents of the parameter pages from the parameter page copies present.
>
> Signed-off-by: Jane Wan <Jane.Wan@nokia.com>
> ---
> v7: change debug print messages
> v6: support the cases that srcbufs are not contiguous
> v5: make the bit-wise majority functon generic
> v4: move the bit-wise majority code in a separate function
> v3: fix warning message detected by kbuild test robot
> v2: rebase the changes on top of v4.17-rc1
>   
> drivers/mtd/nand/raw/nand_base.c |   50 ++++++++++++++++++++++++++++++++++----
>   1 file changed, 45 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/mtd/nand/raw/nand_base.c b/drivers/mtd/nand/raw/nand_base.c
> index 72f3a89..b43b784 100644
> --- a/drivers/mtd/nand/raw/nand_base.c
> +++ b/drivers/mtd/nand/raw/nand_base.c
> @@ -5087,6 +5087,35 @@ static int nand_flash_detect_ext_param_page(struct nand_chip *chip,
>   }
>   
>   /*
> + * Recover data with bit-wise majority
> + */
> +static void nand_bit_wise_majority(const void **srcbufs,
> +				   unsigned int nsrcbufs,
> +				   void *dstbuf,
> +				   unsigned int bufsize)
> +{
> +	int i, j, k;
> +
> +	for (i = 0; i < bufsize; i++) {
> +		u8 cnt, val;
> +
> +		val = 0;
> +		for (j = 0; j < 8; j++) {
> +			cnt = 0;
> +			for (k = 0; k < nsrcbufs; k++) {
> +				const u8 *srcbuf = srcbufs[k];
> +
> +				if (srcbuf[i] & BIT(j))
> +					cnt++;
> +			}
> +			if (cnt > nsrcbufs / 2)
> +				val |= BIT(j);
> +		}
> +		((u8 *)dstbuf)[i] = val;
> +	}
> +}
> +
> +/*
>    * Check if the NAND chip is ONFI compliant, returns 1 if it is, 0 otherwise.
>    */
>   static int nand_flash_detect_onfi(struct nand_chip *chip)
> @@ -5102,7 +5131,7 @@ static int nand_flash_detect_onfi(struct nand_chip *chip)
>   		return 0;
>   
>   	/* ONFI chip: allocate a buffer to hold its parameter page */
> -	p = kzalloc(sizeof(*p), GFP_KERNEL);
> +	p = kzalloc((sizeof(*p) * 3), GFP_KERNEL);
>   	if (!p)
>   		return -ENOMEM;
>   
> @@ -5113,21 +5142,32 @@ static int nand_flash_detect_onfi(struct nand_chip *chip)
>   	}
>   
>   	for (i = 0; i < 3; i++) {
> -		ret = nand_read_data_op(chip, p, sizeof(*p), true);
> +		ret = nand_read_data_op(chip, &p[i], sizeof(*p), true);
>   		if (ret) {
>   			ret = 0;
>   			goto free_onfi_param_page;
>   		}
>   
> -		if (onfi_crc16(ONFI_CRC_BASE, (uint8_t *)p, 254) ==
> +		if (onfi_crc16(ONFI_CRC_BASE, (u8 *)&p[i], 254) ==
>   				le16_to_cpu(p->crc)) {
> +			if (i)
> +				memcpy(p, &p[i], sizeof(*p));
>   			break;
>   		}
>   	}
>   
>   	if (i == 3) {
> -		pr_err("Could not find valid ONFI parameter page; aborting\n");
> -		goto free_onfi_param_page;
> +		const void *srcbufs[3] = {p, p + 1, p + 2};
> +
> +		pr_warn("Could not find a valid ONFI parameter page, trying bit-wise majority to recover it\n");
> +		nand_bit_wise_majority(srcbufs, ARRAY_SIZE(srcbufs), p,
> +				       sizeof(*p));
> +
> +		if (onfi_crc16(ONFI_CRC_BASE, (u8 *)p, 254) !=
> +				le16_to_cpu(p->crc)) {
> +			pr_err("ONFI parameter recovery failed, aborting\n");
> +			goto free_onfi_param_page;
> +		}
>   	}
>   
>   	/* Check version */

This version is still hard coded for a three sample bitwise majority vote.
So why not use the method which I suggested previously for v2 and which 
I repeat below?

The three sample bitwise majority can be implemented without bit level 
manipulation using the identity:
majority3(a, b, c) = (a & b) | (a & c) | (b & c)
This can be factorized slightly to (a & (b | c)) | (b & c)
This enables the operation to be performed 8, 16, 32 or even 64 bits at 
a time depending on the hardware.

This method is not only faster and but also more compact.

Cheers,
Chris