drivers: pwm: rpi_pico: Convert clk calculations to fixed-point

Remove the use of floating point to calculate in
`pwm_rpi_get_cycles_per_sec` to fixed point calculations that result in the
same value.  There is still a division here, which is somewhat slow still,
but the end result about twice fast for the whole path through the led pwm
code.

Even better would be to cache these values, since there is no interface in
this driver to change the configuration past init time.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

drivers/pwm/pwm_rpi_pico.c

@@ -38,15 +38,6 @@ struct pwm_rpi_config {
 	const clock_control_subsys_t clk_id;
 };
 
-static float pwm_rpi_get_clkdiv(const struct device *dev, int slice)
-{
-	const struct pwm_rpi_config *cfg = dev->config;
-
-	/* the divider is a fixed point 8.4 convert to float for use in pico-sdk */
-	return (float)cfg->slice_configs[slice].integral +
-		(float)cfg->slice_configs[slice].frac / 16.0f;
-}
-
 static inline uint32_t pwm_rpi_channel_to_slice(uint32_t channel)
 {
 	return channel / 2;
@@ -68,20 +59,22 @@ static int pwm_rpi_get_cycles_per_sec(const struct device *dev, uint32_t ch, uin
 		return -EINVAL;
 	}
 
+	const struct pwm_rpi_slice_config *slice_config = &cfg->slice_configs[slice];
+
 	ret = clock_control_get_rate(cfg->clk_dev, cfg->clk_id, &pclk);
 	if (ret < 0 || pclk == 0) {
 		return -EINVAL;
 	}
 
-	if (cfg->slice_configs[slice].integral == 0) {
+	if (slice_config->integral == 0) {
 		*cycles = pclk;
 	} else {
 		/* No need to check for divide by 0 since the minimum value of
 		 * pwm_rpi_get_clkdiv is 1
 		 */
-		*cycles = (uint64_t)((float)pclk / pwm_rpi_get_clkdiv(dev, slice));
+		*cycles = (uint64_t)pclk * 16 /
+			((uint64_t)slice_config->integral * 16 + slice_config->frac);
 	}
-
 	return 0;
 }