[U-Boot] [PATCH 2/2] spi: mxc_spi: Update pre and post divider algorithm
Troy Kisky
troy.kisky at boundarydevices.com
Thu May 9 20:04:51 CEST 2013
On 5/9/2013 11:00 AM, Troy Kisky wrote:
> 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 at boundarydevices.com>
>> Signed-off-by: Dirk Behme <dirk.behme at 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(®s->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);
oops, this would also become
post_div = fls(pre_div);
>> + 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
>
More information about the U-Boot
mailing list