Re: [PATCH] spi: tegra20-slink: change chip select action order

[Thierry Reding]

On Tue, Mar 26, 2019 at 03:30:50PM +0100, Randolph Maaßen wrote:
> To transfer via SPI the tegra20-slink driver first sets the command
> register, which contains the chip select value, and after that the
> command2 register, which contains the chip select line. This leads to a
> small spike in the chip selct 0 line between the set of the value and
> the selection of the chip select line.
> 
> This commit changes the order of the register writes so that first the
> chip select line is chosen and then the value is set, removing the
> spike.
> 
> Signed-off-by: Randolph Maaßen <gai...@gaireg.de>
> ---
>  drivers/spi/spi-tegra20-slink.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)

Looks good to me. Adding Sowjanya who has been looking into SPI
recently.

Thierry

> diff --git a/drivers/spi/spi-tegra20-slink.c b/drivers/spi/spi-tegra20-slink.c
> index 1427f343b39a..6d4679126213 100644
> --- a/drivers/spi/spi-tegra20-slink.c
> +++ b/drivers/spi/spi-tegra20-slink.c
> @@ -717,9 +717,6 @@ static int tegra_slink_start_transfer_one(struct 
> spi_device *spi,
>       command2 = tspi->command2_reg;
>       command2 &= ~(SLINK_RXEN | SLINK_TXEN);
>  
> -     tegra_slink_writel(tspi, command, SLINK_COMMAND);
> -     tspi->command_reg = command;
> -
>       tspi->cur_direction = 0;
>       if (t->rx_buf) {
>               command2 |= SLINK_RXEN;
> @@ -729,9 +726,18 @@ static int tegra_slink_start_transfer_one(struct 
> spi_device *spi,
>               command2 |= SLINK_TXEN;
>               tspi->cur_direction |= DATA_DIR_TX;
>       }
> +
> +     /*
> +      * Writing to the command2 register bevore the command register prevents
> +      * a spike in chip_select line 0. This selects the chip_select line
> +      * before changing the chip_select value.
> +      */
>       tegra_slink_writel(tspi, command2, SLINK_COMMAND2);
>       tspi->command2_reg = command2;
>  
> +     tegra_slink_writel(tspi, command, SLINK_COMMAND);
> +     tspi->command_reg = command;
> +
>       if (total_fifo_words > SLINK_FIFO_DEPTH)
>               ret = tegra_slink_start_dma_based_transfer(tspi, t);
>       else
> -- 
> 2.11.0
> 

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