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spi: mxc_spi: Update pre and post divider algorithm

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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>
This commit is contained in:
Dirk Behme 2013-05-11 07:25:54 +02:00 • committed by Stefano Babic
parent 8d4c4ffb95
commit 9a30903b44
1 changed files with 12 additions and 18 deletions
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30
drivers/spi/mxc_spi.c
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@ -128,8 +128,8 @@ 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;
u32 ss_pol = 0, sclkpol = 0, sclkpha = 0;
s32 reg_ctrl, reg_config;
u32 ss_pol = 0, sclkpol = 0, sclkpha = 0, pre_div = 0, post_div = 0;
struct cspi_regs *regs = (struct cspi_regs *)mxcs->base;
if (max_hz == 0) {
@ -147,26 +147,20 @@ 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) {
pre_div = DIV_ROUND_UP(clk_src, 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) {
pre_div = (clk_src - 1) / max_hz;
/* fls(1) = 1, fls(0x80000000) = 32, fls(16) = 5 */
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 >>= post_div;
} else {
post_div = 0;
}
}
@ -174,7 +168,7 @@ 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 - 1);
MXC_CSPICTRL_PREDIV(pre_div);
reg_ctrl = (reg_ctrl & ~MXC_CSPICTRL_POSTDIV(0x0F)) |
MXC_CSPICTRL_POSTDIV(post_div);