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

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Wed, 16 May 2018 09:42:27 +0200
Miquel Raynal <miquel.raynal@bootlin.com> wrote:

> Hi Chris,
> 
> > >>> +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:
> > ...
> > +    p = kzalloc((sizeof(*p) * 3), GFP_KERNEL);
> > ...
> >       for (i = 0; i < 3; i++) {
> > ...
> >       if (i == 3) {
> > ...
> > +        const void *srcbufs[3] = {p, p + 1, p + 2};
> > 
> > Moreover the last of these is difficult to generalize.  
> 
> Indeed, this is something to improve. I think Boris' request was to
> prepare changes like this one, to avoid the situation where the code
> does not scale (like this 'p, p + 1, p + 2').

Yep, here is a quick/untested patch [1] making ONFI param page
detection and recovery more robust by reading more than 3 param pages if
there are more. And that's the reason I want a generic bit-wise
majority helper, not something that only works for 3 copies.

[1]http://code.bulix.org/t21eys-335698