Re: [PATCH v3 2/8] lib/bch: Allow easy bit swapping

[Miquel Raynal <miquel.raynal@bootlin.com>]

Boris Brezillon <boris.brezillon@collabora.com> wrote on Thu, 7 May
2020 13:43:30 +0200:

> On Thu,  7 May 2020 13:00:28 +0200
> Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> 
> > From: Boris Brezillon <boris.brezillon@collabora.com>  
> 
> Hehe. I sent a raw diff and now there's my name on this patch :-).
> Given you debugged/polished it, I'd rather have you take the ownership
> here.
> 
> > 
> > It seems that several hardware ECC engine use a swapped representation
> > of bytes compared to software.  
> 
> Some more details here. I think it has to do with how the ECC engine
> is wired to the NAND controller or the order the bits are passed to the
> HW BCH logic. Given the sunxi NAND controller driver has the same
> bit-swapping, I suspect it's actually pretty common.

I'll add these details.

> 
> > It means that when the software BCH
> > engine is working in conjunction with data generated with hardware, we
> > must swap the bits inside bytes, eg:
> > 
> >     0x0A = b0000_1010 -> b0101_0000 = 0x50
> > 
> > Make it possible just by flipping a boolean in the BCH control  
> 
> 		   ^ by adding a swap_bits boolen to the ...

Yeah I see.

> 
> > structure.  
> 
> Regarding the implementation itself, I came up with something simple
> because I didn't want to spend too much time looking at how things
> could be changed to avoid those swap_bits(). I suspect we can prepare
> the ->a_{mod,pow}_tab tables to avoid that. Ivan, any opinion?
> 
> Note that we can also optimize that in a second step, so Miquel can get
> the rest of the series merged.
> 
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> > Cc: Ivan Djelic <ivan.djelic@parrot.com>
> > ---
> >  include/linux/bch.h |  1 +
> >  lib/bch.c           | 84 ++++++++++++++++++++++++++++++++++++++-------
> >  2 files changed, 72 insertions(+), 13 deletions(-)
> > 
> > diff --git a/include/linux/bch.h b/include/linux/bch.h
> > index 9c35e7cd5890..c42f50cacfdc 100644
> > --- a/include/linux/bch.h
> > +++ b/include/linux/bch.h
> > @@ -51,6 +51,7 @@ struct bch_control {
> >  	int            *cache;
> >  	struct gf_poly *elp;
> >  	struct gf_poly *poly_2t[4];
> > +	bool		swap_bits;  
> 
> This new field should be documented.

Indeed.

> 
> >  };
> >  
> >  struct bch_control *bch_init(int m, int t, unsigned int prim_poly);
> > diff --git a/lib/bch.c b/lib/bch.c
> > index 1091841ac716..81bf8b426eea 100644
> > --- a/lib/bch.c
> > +++ b/lib/bch.c
> > @@ -114,6 +114,49 @@ struct gf_poly_deg1 {
> >  	unsigned int   c[2];
> >  };
> >  
> > +static u8 swap_bits_table[] = {
> > +	0x00, 0x80, 0x40, 0xc0, 0x20, 0xa0, 0x60, 0xe0,
> > +	0x10, 0x90, 0x50, 0xd0, 0x30, 0xb0, 0x70, 0xf0,
> > +	0x08, 0x88, 0x48, 0xc8, 0x28, 0xa8, 0x68, 0xe8,
> > +	0x18, 0x98, 0x58, 0xd8, 0x38, 0xb8, 0x78, 0xf8,
> > +	0x04, 0x84, 0x44, 0xc4, 0x24, 0xa4, 0x64, 0xe4,
> > +	0x14, 0x94, 0x54, 0xd4, 0x34, 0xb4, 0x74, 0xf4,
> > +	0x0c, 0x8c, 0x4c, 0xcc, 0x2c, 0xac, 0x6c, 0xec,
> > +	0x1c, 0x9c, 0x5c, 0xdc, 0x3c, 0xbc, 0x7c, 0xfc,
> > +	0x02, 0x82, 0x42, 0xc2, 0x22, 0xa2, 0x62, 0xe2,
> > +	0x12, 0x92, 0x52, 0xd2, 0x32, 0xb2, 0x72, 0xf2,
> > +	0x0a, 0x8a, 0x4a, 0xca, 0x2a, 0xaa, 0x6a, 0xea,
> > +	0x1a, 0x9a, 0x5a, 0xda, 0x3a, 0xba, 0x7a, 0xfa,
> > +	0x06, 0x86, 0x46, 0xc6, 0x26, 0xa6, 0x66, 0xe6,
> > +	0x16, 0x96, 0x56, 0xd6, 0x36, 0xb6, 0x76, 0xf6,
> > +	0x0e, 0x8e, 0x4e, 0xce, 0x2e, 0xae, 0x6e, 0xee,
> > +	0x1e, 0x9e, 0x5e, 0xde, 0x3e, 0xbe, 0x7e, 0xfe,
> > +	0x01, 0x81, 0x41, 0xc1, 0x21, 0xa1, 0x61, 0xe1,
> > +	0x11, 0x91, 0x51, 0xd1, 0x31, 0xb1, 0x71, 0xf1,
> > +	0x09, 0x89, 0x49, 0xc9, 0x29, 0xa9, 0x69, 0xe9,
> > +	0x19, 0x99, 0x59, 0xd9, 0x39, 0xb9, 0x79, 0xf9,
> > +	0x05, 0x85, 0x45, 0xc5, 0x25, 0xa5, 0x65, 0xe5,
> > +	0x15, 0x95, 0x55, 0xd5, 0x35, 0xb5, 0x75, 0xf5,
> > +	0x0d, 0x8d, 0x4d, 0xcd, 0x2d, 0xad, 0x6d, 0xed,
> > +	0x1d, 0x9d, 0x5d, 0xdd, 0x3d, 0xbd, 0x7d, 0xfd,
> > +	0x03, 0x83, 0x43, 0xc3, 0x23, 0xa3, 0x63, 0xe3,
> > +	0x13, 0x93, 0x53, 0xd3, 0x33, 0xb3, 0x73, 0xf3,
> > +	0x0b, 0x8b, 0x4b, 0xcb, 0x2b, 0xab, 0x6b, 0xeb,
> > +	0x1b, 0x9b, 0x5b, 0xdb, 0x3b, 0xbb, 0x7b, 0xfb,
> > +	0x07, 0x87, 0x47, 0xc7, 0x27, 0xa7, 0x67, 0xe7,
> > +	0x17, 0x97, 0x57, 0xd7, 0x37, 0xb7, 0x77, 0xf7,
> > +	0x0f, 0x8f, 0x4f, 0xcf, 0x2f, 0xaf, 0x6f, 0xef,
> > +	0x1f, 0x9f, 0x5f, 0xdf, 0x3f, 0xbf, 0x7f, 0xff,
> > +};
> > +
> > +static u8 swap_bits(struct bch_control *bch, u8 in)
> > +{
> > +	if (!bch->swap_bits)
> > +		return in;
> > +
> > +	return swap_bits_table[in];
> > +}
> > +
> >  /*
> >   * same as bch_encode(), but process input data one byte at a time
> >   */
> > @@ -126,7 +169,9 @@ static void bch_encode_unaligned(struct bch_control *bch,
> >  	const int l = BCH_ECC_WORDS(bch)-1;
> >  
> >  	while (len--) {
> > -		p = bch->mod8_tab + (l+1)*(((ecc[0] >> 24)^(*data++)) & 0xff);
> > +		u8 tmp = swap_bits(bch, *data++);
> > +
> > +		p = bch->mod8_tab + (l+1)*(((ecc[0] >> 24)^(tmp)) & 0xff);
> >  
> >  		for (i = 0; i < l; i++)
> >  			ecc[i] = ((ecc[i] << 8)|(ecc[i+1] >> 24))^(*p++);
> > @@ -145,10 +190,16 @@ static void load_ecc8(struct bch_control *bch, uint32_t *dst,
> >  	unsigned int i, nwords = BCH_ECC_WORDS(bch)-1;
> >  
> >  	for (i = 0; i < nwords; i++, src += 4)
> > -		dst[i] = (src[0] << 24)|(src[1] << 16)|(src[2] << 8)|src[3];
> > +		dst[i] = ((u32)swap_bits(bch, src[0]) << 24) |
> > +			 ((u32)swap_bits(bch, src[1]) << 16) |
> > +			 ((u32)swap_bits(bch, src[2]) << 8) |
> > +			 swap_bits(bch, src[3]);
> >  
> >  	memcpy(pad, src, BCH_ECC_BYTES(bch)-4*nwords);
> > -	dst[nwords] = (pad[0] << 24)|(pad[1] << 16)|(pad[2] << 8)|pad[3];
> > +	dst[nwords] = ((u32)swap_bits(bch, pad[0]) << 24) |
> > +		      ((u32)swap_bits(bch, pad[1]) << 16) |
> > +		      ((u32)swap_bits(bch, pad[2]) << 8) |
> > +		      swap_bits(bch, pad[3]);
> >  }
> >  
> >  /*
> > @@ -161,15 +212,15 @@ static void store_ecc8(struct bch_control *bch, uint8_t *dst,
> >  	unsigned int i, nwords = BCH_ECC_WORDS(bch)-1;
> >  
> >  	for (i = 0; i < nwords; i++) {
> > -		*dst++ = (src[i] >> 24);
> > -		*dst++ = (src[i] >> 16) & 0xff;
> > -		*dst++ = (src[i] >>  8) & 0xff;
> > -		*dst++ = (src[i] >>  0) & 0xff;
> > +		*dst++ = swap_bits(bch, src[i] >> 24);
> > +		*dst++ = swap_bits(bch, src[i] >> 16);
> > +		*dst++ = swap_bits(bch, src[i] >> 8);
> > +		*dst++ = swap_bits(bch, src[i]);
> >  	}
> > -	pad[0] = (src[nwords] >> 24);
> > -	pad[1] = (src[nwords] >> 16) & 0xff;
> > -	pad[2] = (src[nwords] >>  8) & 0xff;
> > -	pad[3] = (src[nwords] >>  0) & 0xff;
> > +	pad[0] = swap_bits(bch, src[nwords] >> 24);
> > +	pad[1] = swap_bits(bch, src[nwords] >> 16);
> > +	pad[2] = swap_bits(bch, src[nwords] >> 8);
> > +	pad[3] = swap_bits(bch, src[nwords]);
> >  	memcpy(dst, pad, BCH_ECC_BYTES(bch)-4*nwords);
> >  }
> >  
> > @@ -240,7 +291,12 @@ void bch_encode(struct bch_control *bch, const uint8_t *data,
> >  	 */
> >  	while (mlen--) {
> >  		/* input data is read in big-endian format */
> > -		w = r[0]^cpu_to_be32(*pdata++);
> > +		w = cpu_to_be32(*pdata++);
> > +		w = (u32)swap_bits(bch, w) |
> > +		    ((u32)swap_bits(bch, w >> 8) << 8) |
> > +		    ((u32)swap_bits(bch, w >> 16) << 16) |
> > +		    ((u32)swap_bits(bch, w >> 24) << 24);  
> 
> We should make the bit swapping conditional (only do it if
> bch->swap_bit is true) to not penalize the !swap_bits case.

Sure

> 
> > +		w ^= r[0];
> >  		p0 = tab0 + (l+1)*((w >>  0) & 0xff);
> >  		p1 = tab1 + (l+1)*((w >>  8) & 0xff);
> >  		p2 = tab2 + (l+1)*((w >> 16) & 0xff);
> > @@ -1048,7 +1104,9 @@ int bch_decode(struct bch_control *bch, const uint8_t *data, unsigned int len,
> >  				break;
> >  			}
> >  			errloc[i] = nbits-1-errloc[i];
> > -			errloc[i] = (errloc[i] & ~7)|(7-(errloc[i] & 7));
> > +			if (!bch->swap_bits)
> > +				errloc[i] = (errloc[i] & ~7) |
> > +					    (7-(errloc[i] & 7));
> >  		}
> >  	}
> >  	return (err >= 0) ? err : -EBADMSG;  
> 
> Oh, and we should patch bch_init() to take a bool swap_bits. I'm not
> comfortable having users set bch_control fields on their own,
> especially one that's marked 'private'.

Oh yeah, I had this in mind but I completely forgot. I'll do it.