[PATCH 4/4] spi: bcm2835aux: fix CPOL/CPHA setting

[Eric Anholt]

stephanolbrich at gmx.de writes:

> From: Stephan Olbrich <stephanolbrich at gmx.de>
>
> The auxiliary spi supports only CPHA=0 modes as the first bit is
> always output to the pin before the first clock cycle. In CPHA=1
> modes the first clock edge outputs the second bit hence the slave
> can never read the first bit.
>
> Also the CPHA registers switch between clocking data in/out on
> rising/falling edge hence depend on the CPOL setting.
>
> Signed-off-by: Stephan Olbrich <stephanolbrich at gmx.de>
> ---
>  drivers/spi/spi-bcm2835aux.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/spi/spi-bcm2835aux.c b/drivers/spi/spi-bcm2835aux.c
> index b90aa34..169f521 100644
> --- a/drivers/spi/spi-bcm2835aux.c
> +++ b/drivers/spi/spi-bcm2835aux.c
> @@ -386,12 +386,12 @@ static int bcm2835aux_spi_prepare_message(struct spi_master *master,
>  	bs->cntl[1] = BCM2835_AUX_SPI_CNTL1_MSBF_IN;
>  
>  	/* handle all the modes */
> -	if (spi->mode & SPI_CPOL)
> +	if (spi->mode & SPI_CPOL) {
>  		bs->cntl[0] |= BCM2835_AUX_SPI_CNTL0_CPOL;
> -	if (spi->mode & SPI_CPHA)
> -		bs->cntl[0] |= BCM2835_AUX_SPI_CNTL0_CPHA_OUT |
> -			       BCM2835_AUX_SPI_CNTL0_CPHA_IN;
> -
> +		bs->cntl[0] |= BCM2835_AUX_SPI_CNTL0_CPHA_OUT;
> +	} else {
> +		bs->cntl[0] |= BCM2835_AUX_SPI_CNTL0_CPHA_IN;
> +	}
>  	bcm2835aux_wr(bs, BCM2835_AUX_SPI_CNTL1, bs->cntl[1]);
>  	bcm2835aux_wr(bs, BCM2835_AUX_SPI_CNTL0, bs->cntl[0]);

(Note for other readers: A better name for CNTL0_CPHA_* would be
CNTL0_*_RISING).

I think you're right about not actually supporting CPHA.  I don't see
wany way to keep bit 1 out lasting through the first full clock cycle.
I think Stefan's right that we should drop CPHA from MODE_BITS
(actually, MODE_BITS would be nicer if we just merged it into its one
user, I think).

However, this hunk appears to be correct and would fix the timing of our
data going out in the CPOL=0 case and of sampling IN data for CPOL=1.
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