The change fixes a problem, if duty_ns is too small in comparison
to period_ns (as a valid corner case duty_ns is 0 ns), then due to
PWM_DUTY() macro applied on a value the result is overflowed over 8
bits, and instead of the highest bitfield duty cycle value 0xff the
invalid duty cycle bitfield value 0x00 is written.
For reference the LPC32xx spec defines PWMx_DUTY bitfield description
is this way and it seems to be correct:
[Low]/[High] = [PWM_DUTY]/[256-PWM_DUTY], where 0 < PWM_DUTY <= 255.
In addition according to my oscilloscope measurements LPC32xx PWM is
"tristate" in sense that it produces a wave with floating min/max
voltage levels for different duty cycle values, for corner cases:
PWM_DUTY == 0x01 => signal is in range from -1.05v to 0v
....
PWM_DUTY == 0x80 => signal is in range from -0.75v to +0.75v
....
PWM_DUTY == 0xff => signal is in range from 0v to +1.05v
PWM_DUTY == 0x00 => signal is around 0v, PWM is off
Due to this peculiarity on very long period ranges (less than 1KHz)
and odd pre-divider values PWM generated wave does not remind a
clock shape signal, but rather a heartbit shape signal with positive
and negative peaks, so I would recommend to use high-speed HCLK clock
as a PWM parent clock and avoid using RTC clock as a parent.
The change corrects PWM output in corner cases and prevents any
possible overflows in calculation of values for PWM_DUTY and
PWM_RELOADV bitfields, thus helper macro definitions may be removed.
Signed-off-by: Vladimir Zapolskiy <vz@xxxxxxxxx>
---
Changes from v1 to v2:
- none
drivers/pwm/pwm-lpc32xx.c | 53 +++++++++++++++++------------------------------
1 file changed, 19 insertions(+), 34 deletions(-)
diff --git a/drivers/pwm/pwm-lpc32xx.c b/drivers/pwm/pwm-lpc32xx.c
index 63468a8..294a68f 100644
--- a/drivers/pwm/pwm-lpc32xx.c
+++ b/drivers/pwm/pwm-lpc32xx.c
@@ -24,9 +24,7 @@ struct lpc32xx_pwm_chip {
void __iomem *base;
};
-#define PWM_ENABLE (1 << 31)
-#define PWM_RELOADV(x) (((x) & 0xFF) << 8)
-#define PWM_DUTY(x) ((x) & 0xFF)
+#define PWM_ENABLE BIT(31)
#define to_lpc32xx_pwm_chip(_chip) \
container_of(_chip, struct lpc32xx_pwm_chip, chip)
@@ -38,40 +36,27 @@ static int lpc32xx_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
unsigned long long c;
int period_cycles, duty_cycles;
u32 val;
-
- c = clk_get_rate(lpc32xx->clk) / 256;
- c = c * period_ns;
- do_div(c, NSEC_PER_SEC);
-
- /* Handle high and low extremes */
- if (c == 0)
- c = 1;
- if (c > 255)
- c = 0; /* 0 set division by 256 */
- period_cycles = c;
-
- /* The duty-cycle value is as follows:
- *
- * DUTY-CYCLE HIGH LEVEL
- * 1 99.9%
- * 25 90.0%
- * 128 50.0%
- * 220 10.0%
- * 255 0.1%
- * 0 0.0%
- *
- * In other words, the register value is duty-cycle % 256 with
- * duty-cycle in the range 1-256.
- */
- c = 256 * duty_ns;
- do_div(c, period_ns);
- if (c > 255)
- c = 255;
- duty_cycles = 256 - c;
+ c = clk_get_rate(lpc32xx->clk);
+
+ /* The highest acceptable divisor is 256, which is represented by 0 */
+ period_cycles = div64_u64(c * period_ns,
+ (unsigned long long)NSEC_PER_SEC * 256);
+ if (!period_cycles)
+ period_cycles = 1;
+ if (period_cycles > 255)
+ period_cycles = 0;
+
+ /* Compute 256 x #duty/period value and care for corner cases */
+ duty_cycles = div64_u64((unsigned long long)(period_ns - duty_ns) * 256,
+ period_ns);
+ if (!duty_cycles)
+ duty_cycles = 1;
+ if (duty_cycles > 255)
+ duty_cycles = 255;
val = readl(lpc32xx->base + (pwm->hwpwm << 2));
val &= ~0xFFFF;
- val |= PWM_RELOADV(period_cycles) | PWM_DUTY(duty_cycles);
+ val |= (period_cycles << 8) | duty_cycles;
writel(val, lpc32xx->base + (pwm->hwpwm << 2));
return 0;
--
2.1.4
--
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