spi: mxc_spi: Update pre and post divider algorithm
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>
diff --git a/drivers/spi/mxc_spi.c b/drivers/spi/mxc_spi.c
index b553a9c..0eca776 100644
--- a/drivers/spi/mxc_spi.c
+++ b/drivers/spi/mxc_spi.c
@@ -128,8 +128,8 @@
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 @@
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) {
- 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 @@
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);