Re: [PATCH v5 1/3] mtd: nand: gpmi: add gpmi_move_bits function

[Boris Brezillon]

On Wed, 26 Nov 2014 17:53:11 +0100
Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote:

> Add a new function to move bits (not bytes) from a memory region to
> another one.
> This function is similar to memmove except it acts at bit level.
> This function is needed to implement GPMI raw access functions, given the
> fact that ECC engine does not pad ECC bits to the next byte boundary.
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>
> ---
>  drivers/mtd/nand/gpmi-nand/gpmi-lib.c  | 151 +++++++++++++++++++++++++++++++++
>  drivers/mtd/nand/gpmi-nand/gpmi-nand.h |   4 +
>  2 files changed, 155 insertions(+)
> 
> diff --git a/drivers/mtd/nand/gpmi-nand/gpmi-lib.c b/drivers/mtd/nand/gpmi-nand/gpmi-lib.c
> index 87e658c..77eff0e 100644
> --- a/drivers/mtd/nand/gpmi-nand/gpmi-lib.c
> +++ b/drivers/mtd/nand/gpmi-nand/gpmi-lib.c
> @@ -1353,3 +1353,154 @@ int gpmi_read_page(struct gpmi_nand_data *this,
>  	set_dma_type(this, DMA_FOR_READ_ECC_PAGE);
>  	return start_dma_with_bch_irq(this, desc);
>  }
> +
> +/**
> + * gpmi_move_bits - move bits from one memory region to another
> + * @dst: destination buffer
> + * @dst_bit_off: bit offset we're starting to write at
> + * @src: source buffer
> + * @src_bit_off: bit offset we're starting to read from
> + * @nbits: number of bits to copy
> + *
> + * This functions copies bits from one memory region to another, and is used by
> + * the GPMI driver to copy ECC sections which are not guaranteed to be byte
> + * aligned.

I forgot to add that src and dst should not overlap, otherwise it won't
work...

> + *
> + */
> +void gpmi_move_bits(u8 *dst, size_t dst_bit_off,
> +		    const u8 *src, size_t src_bit_off,
> +		    size_t nbits)
> +{
> +	size_t i;
> +	size_t nbytes;
> +	u32 src_buffer = 0;
> +	size_t bits_in_src_buffer = 0;
> +
> +	if (!nbits)
> +		return;
> +
> +	/*
> +	 * Move src and dst pointers to the closest byte pointer and store bit
> +	 * offsets within a byte.
> +	 */
> +	src += src_bit_off / 8;
> +	src_bit_off %= 8;
> +
> +	dst += dst_bit_off / 8;
> +	dst_bit_off %= 8;
> +
> +	/*
> +	 * Initialize the src_buffer value with bits available in the first
> +	 * byte of data so that we end up with a byte aligned src pointer.
> +	 */
> +	if (src_bit_off) {
> +		src_buffer = src[0] >> src_bit_off;
> +		if (nbits >= (8 - src_bit_off)) {
> +			bits_in_src_buffer += 8 - src_bit_off;
> +		} else {
> +			src_buffer &= GENMASK(nbits - 1, 0);
> +			bits_in_src_buffer += nbits;
> +		}
> +		nbits -= bits_in_src_buffer;
> +		src++;
> +	}
> +
> +	/* Calculate the number of bytes that can be copied from src to dst. */
> +	nbytes = nbits / 8;
> +
> +	/* Try to align dst to a byte boundary. */
> +	if (dst_bit_off) {
> +		if (bits_in_src_buffer < (8 - dst_bit_off) && nbytes) {
> +			src_buffer |= src[0] << bits_in_src_buffer;
> +			bits_in_src_buffer += 8;
> +			src++;
> +			nbytes--;
> +		}
> +
> +		if (bits_in_src_buffer >= (8 - dst_bit_off)) {
> +			dst[0] &= GENMASK(dst_bit_off - 1, 0);
> +			dst[0] |= src_buffer << dst_bit_off;
> +			src_buffer >>= (8 - dst_bit_off);
> +			bits_in_src_buffer -= (8 - dst_bit_off);
> +			dst_bit_off = 0;
> +			dst++;
> +			if (bits_in_src_buffer > 7) {
> +				bits_in_src_buffer -= 8;
> +				dst[0] = src_buffer;
> +				dst++;
> +				src_buffer >>= 8;
> +			}
> +		}
> +	}
> +
> +	if (!bits_in_src_buffer && !dst_bit_off) {
> +		/*
> +		 * Both src and dst pointers are byte aligned, thus we can
> +		 * just use the optimized memcpy function.
> +		 */
> +		if (nbytes)
> +			memcpy(dst, src, nbytes);
> +	} else {
> +		/*
> +		 * src buffer is not byte aligned, hence we have to copy each
> +		 * src byte to the src_buffer variable before extracting a byte
> +		 * to store in dst.
> +		 */
> +		for (i = 0; i < nbytes; i++) {
> +			src_buffer |= src[i] << bits_in_src_buffer;
> +			dst[i] = src_buffer;
> +			src_buffer >>= 8;
> +		}
> +	}
> +	/* Update dst and src pointers */
> +	dst += nbytes;
> +	src += nbytes;
> +
> +	/*
> +	 * nbits is the number of remaining bits. It should not exceed 8 as
> +	 * we've already copied as much bytes as possible.
> +	 */
> +	nbits %= 8;
> +
> +	/*
> +	 * If there's no more bits to copy to the destination and src buffer
> +	 * was already byte aligned, then we're done.
> +	 */
> +	if (!nbits && !bits_in_src_buffer)
> +		return;
> +
> +	/* Copy the remaining bits to src_buffer */
> +	if (nbits)
> +		src_buffer |= (*src & GENMASK(nbits - 1, 0)) <<
> +			      bits_in_src_buffer;
> +	bits_in_src_buffer += nbits;
> +
> +	/*
> +	 * In case there were not enough bits to get a byte aligned dst buffer
> +	 * prepare the src_buffer variable to match the dst organization (shift
> +	 * src_buffer by dst_bit_off and retrieve the least significant bits
> +	 * from dst).
> +	 */
> +	if (dst_bit_off)
> +		src_buffer = (src_buffer << dst_bit_off) |
> +			     (*dst & GENMASK(dst_bit_off - 1, 0));
> +	bits_in_src_buffer += dst_bit_off;
> +
> +	/*
> +	 * Keep most significant bits from dst if we end up with an unaligned
> +	 * number of bits.
> +	 */
> +	nbytes = bits_in_src_buffer / 8;
> +	if (bits_in_src_buffer % 8) {
> +		src_buffer |= (dst[nbytes] &
> +			       GENMASK(7, bits_in_src_buffer % 8)) <<
> +			      (nbytes * 8);
> +		nbytes++;
> +	}
> +
> +	/* Copy the remaining bytes to dst */
> +	for (i = 0; i < nbytes; i++) {
> +		dst[i] = src_buffer;
> +		src_buffer >>= 8;
> +	}
> +}
> diff --git a/drivers/mtd/nand/gpmi-nand/gpmi-nand.h b/drivers/mtd/nand/gpmi-nand/gpmi-nand.h
> index 32c6ba4..17d0736 100644
> --- a/drivers/mtd/nand/gpmi-nand/gpmi-nand.h
> +++ b/drivers/mtd/nand/gpmi-nand/gpmi-nand.h
> @@ -290,6 +290,10 @@ extern int gpmi_send_page(struct gpmi_nand_data *,
>  extern int gpmi_read_page(struct gpmi_nand_data *,
>  			dma_addr_t payload, dma_addr_t auxiliary);
>  
> +void gpmi_move_bits(u8 *dst, size_t dst_bit_off,
> +		    const u8 *src, size_t src_bit_off,
> +		    size_t nbits);
> +
>  /* BCH : Status Block Completion Codes */
>  #define STATUS_GOOD		0x00
>  #define STATUS_ERASED		0xff



-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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