[PATCH] spi: bcm2835: set up spi-mode before asserting cs-gpio

[Martin Sperl]

Any feedback on this patch? Can this get merged?

Noralf (via private communication) has confirmed that with
this patch his fbtft device (which uses spi-cpol and spi-cpha)
works.

Thanks,
	Martin

On 28.07.2015 16:03, kernel at martin.sperl.org wrote:
> From: Martin Sperl <kernel at martin.sperl.org>
>
> When using reverse polarity for clock (spi-cpol) on a device
> the clock line gets altered after chip-select has been asserted
> resulting in an additional clock beat, which confuses hardware.
>
> This did not show when using native-CS, as the same register
> is used to control cs as well as polarity, so the changes came
> into effect at the same time. Unfortunately this is not true
> with gpio-cs.
>
> To avoid this situation this patch moves the setup of polarity
> (spi-cpol and spi-cpha) outside of the chip-select into
> prepare_message, which is run prior to asserting chip-select.
>
> Also fixes resetting 3-wire mode after use of rx-mode, so that
> a 3-Wire sequence TX, RX, TX works as well (right now it runs
> TX, RX, RX instead)
>
> Reported-by: Noralf Tronnes <noralf at tronnes.org>
> Signed-off-by: Martin Sperl <kernel at martin.sperl.org>
> ---
>   drivers/spi/spi-bcm2835.c |   28 +++++++++++++++++++++++-----
>   1 file changed, 23 insertions(+), 5 deletions(-)
>
> See also the images for the effect that this patch has on the signals:
> https://github.com/raspberrypi/linux/pull/1077#issuecomment-125484500
>
> diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
> index 59705ab..c9357bb 100644
> --- a/drivers/spi/spi-bcm2835.c
> +++ b/drivers/spi/spi-bcm2835.c
> @@ -553,13 +553,11 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
>   	spi_used_hz = cdiv ? (clk_hz / cdiv) : (clk_hz / 65536);
>   	bcm2835_wr(bs, BCM2835_SPI_CLK, cdiv);
>
> -	/* handle all the modes */
> +	/* handle all the 3-wire mode */
>   	if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf))
>   		cs |= BCM2835_SPI_CS_REN;
> -	if (spi->mode & SPI_CPOL)
> -		cs |= BCM2835_SPI_CS_CPOL;
> -	if (spi->mode & SPI_CPHA)
> -		cs |= BCM2835_SPI_CS_CPHA;
> +	else
> +		cs &= ~BCM2835_SPI_CS_REN;
>
>   	/* for gpio_cs set dummy CS so that no HW-CS get changed
>   	 * we can not run this in bcm2835_spi_set_cs, as it does
> @@ -592,6 +590,25 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
>   	return bcm2835_spi_transfer_one_irq(master, spi, tfr, cs);
>   }
>
> +static int bcm2835_spi_prepare_message(struct spi_master *master,
> +				       struct spi_message *msg)
> +{
> +	struct spi_device *spi = msg->spi;
> +	struct bcm2835_spi *bs = spi_master_get_devdata(master);
> +	u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
> +
> +	cs &= ~(BCM2835_SPI_CS_CPOL | BCM2835_SPI_CS_CPHA);
> +
> +	if (spi->mode & SPI_CPOL)
> +		cs |= BCM2835_SPI_CS_CPOL;
> +	if (spi->mode & SPI_CPHA)
> +		cs |= BCM2835_SPI_CS_CPHA;
> +
> +	bcm2835_wr(bs, BCM2835_SPI_CS, cs);
> +
> +	return 0;
> +}
> +
>   static void bcm2835_spi_handle_err(struct spi_master *master,
>   				   struct spi_message *msg)
>   {
> @@ -739,6 +756,7 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
>   	master->set_cs = bcm2835_spi_set_cs;
>   	master->transfer_one = bcm2835_spi_transfer_one;
>   	master->handle_err = bcm2835_spi_handle_err;
> +	master->prepare_message = bcm2835_spi_prepare_message;
>   	master->dev.of_node = pdev->dev.of_node;
>
>   	bs = spi_master_get_devdata(master);
>