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

[Chris Moore]

Hi,

Le 15/05/2018 Ã 09:34, Boris Brezillon a ÃcritÂ:
> On Tue, 15 May 2018 06:45:51 +0200
> Chris Moore <moore@xxxxxxx> wrote:
>
> > Hi,
> >
> > Le 13/05/2018 Ã 06:30, Wan, Jane (Nokia - US/Sunnyvale) a ÃcritÂ:
> > > 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@xxxxxxxxx>
> > > ---
> > > 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?
> Because I want the nand_bit_wise_majority() function to work with
> nsrcbufs > 3 (the ONFI spec says there's at least 3 copy of the param
> page, but NAND vendor can decide to put more). Also, if the X copies of
> the PARAM are corrupted (which is rather unlikely), that means we
> already spent quite a lot of time reading the different copies and
> calculating the CRC, so I think we don't care about perf optimizations
> when doing bit-wise majority.
>
> > 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.

I do understand that the ONFI specifications permit more than 3 copies.
However elsewhere the proposed code shows no intention of handling other 
cases.
The constant 3 is hard coded in the following lines extracted from the 
proposed code:
...
+ÂÂÂ p = kzalloc((sizeof(*p) * 3), GFP_KERNEL);
...
ÂÂÂÂ for (i = 0; i < 3; i++) {
...
ÂÂÂÂ if (i == 3) {
...
+ÂÂÂ ÂÂÂ const void *srcbufs[3] = {p, p + 1, p + 2};

Moreover the last of these is difficult to generalize.

Cheers,
Chris