[U-Boot] [PATCH 2/2] spi: mxc_spi: Update pre and post divider algorithm

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

On 5/8/2013 10:19 PM, Dirk Behme wrote:
> The spi clock divisor is of the form x * (2**y),  or  x  << y, where x is
> 1 to 16, and y is 0 to 15. Note the similarity with floating point numbers.
> Convert the desired divisor to the smallest number which is >= desired divisor,
> and can be represented in this form. The previous algorithm chose a divisor
> which could be almost twice as large as needed.
>
> Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
> Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
> ---
>   drivers/spi/mxc_spi.c |   27 ++++++++++++---------------
>   1 file changed, 12 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/spi/mxc_spi.c b/drivers/spi/mxc_spi.c
> index 3e903b3..66c2ad8 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 = 1, post_div = 0, i, reg_ctrl, reg_config;
> +	s32 pre_div = 1, post_div = 0, reg_ctrl, reg_config;

First, I'm totally fine with the patch as it is. I'm just going to point 
out things you may want to change, or
send a follow-up patch.

Here, no need to initialize pre_div, post_div, if you delete the  if 
(clk_src > max_hz) below which is not needed.

>   	u32 ss_pol = 0, sclkpol = 0, sclkpha = 0;
>   	struct cspi_regs *regs = (struct cspi_regs *)mxcs->base;
>   
> @@ -147,27 +147,24 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave *mxcs, unsigned int cs,
>   	reg_ctrl |=  MXC_CSPICTRL_EN;
>   	reg_write(&regs->ctrl, reg_ctrl);
>   
> -	/*
> -	 * The following computation is taken directly from Freescale's code.
> -	 */
>   	if (clk_src > max_hz) {
This "if" can be removed.

>   		pre_div = DIV_ROUND_UP(clk_src, max_hz);
If you subtract -1 here instead of when you set the divisor register, 
the logic becomes simpler


pre_div = DIV_ROUND_UP(clk_src, max_hz) - 1;

or just

pre_div = (clk_src - 1) / max_hz;

> -		if (pre_div > 16) {
> -			post_div = pre_div / 16;
> -			pre_div = 16;
> -		}
> -		if (post_div != 0) {
> -			for (i = 0; i < 16; i++) {
> -				if ((1 << i) >= post_div)
> -					break;
> -			}
> -			if (i == 16) {
> +		/* fls(1) = 1, fls(0x80000000) = 32, fls(16) = 5 */
> +		post_div = fls(pre_div - 1);
> +		if (post_div > 4) {
> +			post_div -= 4;
> +
> +			if (post_div >= 16) {
>   				printf("Error: no divider for the freq: %d\n",
>   					max_hz);
>   				return -1;
>   			}
> -			post_div = i;
> +			pre_div = (pre_div + (1 << post_div) - 1) >> post_div;

This would become
pre_div >>= post_div;
> +
> +		} else {
> +			post_div = 0;
>   		}
> +
>   	}
>   
>   	debug("pre_div = %d, post_div=%d\n", pre_div, post_div);

And

MXC_CSPICTRL_PREDIV(pre_div - 1);

would return to

MXC_CSPICTRL_PREDIV(pre_div);


Troy