Re: [PATCH 2/3] spi: bitbang: add lsb first support

[Gerhard Sittig <gsi-ynQEQJNshbs@public.gmane.org>]

On Wed, Mar 12, 2014 at 16:20 +0100, Michael Grzeschik wrote:
> 
> The bitbang spi driver currently only supports the MSB mode. This patch
> adds the possibility to clock the data in LSB mode.
> 
> Signed-off-by: Michael Grzeschik <m.grzeschik-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>
> ---
>  drivers/spi/spi-bitbang-txrx.h | 98 +++++++++++++++++++++++++++++-------------
>  drivers/spi/spi-bitbang.c      |  3 +-
>  2 files changed, 71 insertions(+), 30 deletions(-)
> 
> diff --git a/drivers/spi/spi-bitbang-txrx.h b/drivers/spi/spi-bitbang-txrx.h
> index b6e348d..7f9c020 100644
> --- a/drivers/spi/spi-bitbang-txrx.h
> +++ b/drivers/spi/spi-bitbang-txrx.h
> @@ -49,22 +49,42 @@ bitbang_txrx_be_cpha0(struct spi_device *spi,
>  {
>  	/* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */
>  
> -	/* clock starts at inactive polarity */
> -	for (word <<= (32 - bits); likely(bits); bits--) {
> -
> -		/* setup MSB (to slave) on trailing edge */
> -		if ((flags & SPI_MASTER_NO_TX) == 0)
> -			setmosi(spi, word & (1 << 31));
> -		spidelay(nsecs);	/* T(setup) */
> -
> -		setsck(spi, !cpol);
> -		spidelay(nsecs);
> -
> -		/* sample MSB (from slave) on leading edge */
> -		if ((flags & SPI_MASTER_NO_RX) == 0)
> -			word |= getmiso(spi);
> -		setsck(spi, cpol);
> -		word <<= 1;
> +	if (spi->mode & SPI_LSB_FIRST) {
> +		/* clock starts at inactive polarity */
> +		for (; likely(bits); bits--) {
> +
> +			/* setup MSB (to slave) on trailing edge */
> +			if ((flags & SPI_MASTER_NO_TX) == 0)
> +				setmosi(spi, word & 1);
> +			spidelay(nsecs);	/* T(setup) */
> +
> +			setsck(spi, !cpol);
> +			spidelay(nsecs);
> +
> +			/* sample LSB (from slave) on leading edge */
> +			if ((flags & SPI_MASTER_NO_RX) == 0)
> +				word |= getmiso(spi);
> +			setsck(spi, cpol);
> +			word >>= 1;
> +		}
> +	} else {
> +		/* clock starts at inactive polarity */
> +		for (word <<= (32 - bits); likely(bits); bits--) {
> +
> +			/* setup MSB (to slave) on trailing edge */
> +			if ((flags & SPI_MASTER_NO_TX) == 0)
> +				setmosi(spi, word & (1 << 31));
> +			spidelay(nsecs);        /* T(setup) */
> +
> +			setsck(spi, !cpol);
> +			spidelay(nsecs);
> +
> +			/* sample MSB (from slave) on leading edge */
> +			if ((flags & SPI_MASTER_NO_RX) == 0)
> +				word |= getmiso(spi);
> +			setsck(spi, cpol);
> +			word <<= 1;
> +		}
>  	}
>  	return word;
>  }

Would it be useful to not duplicate the transmission logic, but
instead to optionally "bit swap" the TX and RX data before and
after the common transmission logic?  Just a though whether this
might help maintenance.  There might even be existing and proven
code to bit swap integers?


virtually yours
Gerhard Sittig
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