[U-Boot] [PATCH] spi: mxc_spi: Fix pre and post divider calculation

[Troy Kisky <troy.kisky@boundarydevices.com>]

On 5/2/2013 3:59 AM, Dirk Behme wrote:
> From: Dirk Behme <dirk.behme@gmail.com>
>
> Fix two issues with the calculation of pre_div and post_div:
>
> 1. pre_div: While the calculation of pre_div looks correct, to set the
> CONREG[15-12] bits pre_div needs to be decremented by 1:
>
> The i.MX 6Dual/6Quad Applications Processor Reference Manual (IMX6DQRM
> Rev. 0, 11/2012) states:
>
> CONREG[15-12]: PRE_DIVIDER
> 0000 Divide by 1
> 0001 Divide by 2
> 0010 Divide by 3
> ...
> 1101 Divide by 14
> 1110 Divide by 15
> 1111 Divide by 16
>
> I.e. if we want to divide by 2, we have to write 1 to CONREG[15-12].
>
> 2. In case the post divider becomes necessary, pre_div will be set to 15. To
> calculate the post divider, divide by 15, too. And not 16.
>
> Both issues above are tested using the following examples:
>
> clk_src = 60000000 (60MHz, default i.MX6 ECSPI clock)
>
> a) max_hz == 23000000 (23MHz, max i.MX6 ECSPI read clock)
>
> -> pre_div =  3 (divide by 3 => CONREG[15-12] == 2)
> -> post_div = 0 (divide by 1 => CONREG[11- 8] == 0)
>                 => 60MHz / 3 = 20MHz SPI clock
>
> b) max_hz == 2000000 (2MHz)
>
> -> pre_div =  15 (divide by 15 => CONREG[15-12] == 14)
> -> post_div = 1  (divide by  2 => CONREG[11- 8] == 1)
>                 => 60MHz / 30 = 2MHz SPI clock
>
> c) max_hz == 1000000 (1MHz)
>
> -> pre_div =  15 (divide by 15 => CONREG[15-12] == 14)
> -> post_div = 2  (divide by  4 => CONREG[11- 8] == 2)
>                 => 60MHz / 60 = 1MHz SPI clock
>
> d) max_hz == 500000 (500kHz)
>
> -> pre_div =  15 (divide by 15 => CONREG[15-12] == 14)
> -> post_div = 3  (divide by  8 => CONREG[11- 8] == 3)
>                 => 60MHz / 120 = 500kHz SPI clock
>
> Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
> ---
>   drivers/spi/mxc_spi.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/spi/mxc_spi.c b/drivers/spi/mxc_spi.c
> index 5bed858..8630bbd 100644
> --- a/drivers/spi/mxc_spi.c
> +++ b/drivers/spi/mxc_spi.c
> @@ -128,7 +128,7 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave *mxcs, unsigned int cs,
>   		unsigned int max_hz, unsigned int mode)
>   {
>   	u32 clk_src = mxc_get_clock(MXC_CSPI_CLK);
> -	s32 pre_div = 0, post_div = 0, i, reg_ctrl, reg_config;
> +	s32 pre_div = 1, post_div = 0, i, reg_ctrl, reg_config;
>   	u32 ss_pol = 0, sclkpol = 0, sclkpha = 0;
>   	struct cspi_regs *regs = (struct cspi_regs *)mxcs->base;
>   
> @@ -153,7 +153,7 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave *mxcs, unsigned int cs,
>   	if (clk_src > max_hz) {
>   		pre_div = DIV_ROUND_UP(clk_src, max_hz);
>   		if (pre_div > 16) {
> -			post_div = pre_div / 16;
> +			post_div = pre_div / 15;
I think this should not change
>   			pre_div = 15;
and this should be = 16, because you now subtract 1 below
>   		}
>   		if (post_div != 0) {
> @@ -174,7 +174,7 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave *mxcs, unsigned int cs,
>   	reg_ctrl = (reg_ctrl & ~MXC_CSPICTRL_SELCHAN(3)) |
>   		MXC_CSPICTRL_SELCHAN(cs);
>   	reg_ctrl = (reg_ctrl & ~MXC_CSPICTRL_PREDIV(0x0F)) |
> -		MXC_CSPICTRL_PREDIV(pre_div);
> +		MXC_CSPICTRL_PREDIV(pre_div - 1);
>   	reg_ctrl = (reg_ctrl & ~MXC_CSPICTRL_POSTDIV(0x0F)) |
>   		MXC_CSPICTRL_POSTDIV(post_div);
>