[PATCH 7/8] pwm: stm32: use input prescaler to improve period capture

Fabrice Gasnier
2018-01-16T12:43:50Z

Using input prescaler, capture unit will trigger DMA once every
configurable /2, /4 or /8 events (rising edge). This can help improve
period (only) capture accuracy.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier-qxv4g6HH51o@public.gmane.org>
---
drivers/pwm/pwm-stm32.c | 67 ++++++++++++++++++++++++++++++++++++++--
include/linux/mfd/stm32-timers.h | 1 +
2 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c
index c890404..6b99d92 100644
--- a/drivers/pwm/pwm-stm32.c
+++ b/drivers/pwm/pwm-stm32.c
@@ -9,6 +9,7 @@
* pwm-atmel.c from Bo Shen
*/

+#include <linux/bitfield.h>
#include <linux/dma-mapping.h>
#include <linux/mfd/stm32-timers.h>
#include <linux/module.h>
@@ -257,7 +258,7 @@ static int stm32_pwm_capture(struct pwm_chip *chip, struct pwm_device *pwm,
struct stm32_pwm *priv = to_stm32_pwm_dev(chip);
unsigned long long prd, div, dty;
unsigned long rate;
- unsigned int psc = 0, scale;
+ unsigned int psc = 0, icpsc, scale;
u32 raw_prd, raw_dty;
int ret = 0;

@@ -314,6 +315,7 @@ static int stm32_pwm_capture(struct pwm_chip *chip, struct pwm_device *pwm,
/*
* Got a capture. Try to improve accuracy at high rates:
* - decrease counter clock prescaler, scale up to max rate.
+ * - use input prescaler, capture once every /2 /4 or /8 edges.
*/
if (raw_prd) {
u32 max_arr = priv->max_arr - 0x1000; /* arbitrary margin */
@@ -335,8 +337,69 @@ static int stm32_pwm_capture(struct pwm_chip *chip, struct pwm_device *pwm,
raw_dty = priv->capture[1];
}

+ /* Compute intermediate period not to exceed timeout at low rates */
prd = (unsigned long long)raw_prd * (psc + 1) * NSEC_PER_SEC;
- result->period = DIV_ROUND_UP_ULL(prd, rate);
+ do_div(prd, rate);
+
+ for (icpsc = 0; icpsc < MAX_TIM_ICPSC ; icpsc++) {
+ /* input prescaler: also keep arbitrary margin */
+ if (raw_prd >= (priv->max_arr - 0x1000) >> (icpsc + 1))
+ break;
+ if (prd >= (tmo_ms * NSEC_PER_MSEC) >> (icpsc + 2))
+ break;
+ }
+
+ if (!icpsc)
+ goto done;
+
+ /* Last chance to improve period accuracy, using input prescaler */
+ regmap_update_bits(priv->regmap,
+ pwm->hwpwm < 2 ? TIM_CCMR1 : TIM_CCMR2,
+ TIM_CCMR_IC1PSC | TIM_CCMR_IC2PSC,
+ FIELD_PREP(TIM_CCMR_IC1PSC, icpsc) |
+ FIELD_PREP(TIM_CCMR_IC2PSC, icpsc));
+
+ ret = stm32_pwm_do_capture(priv, pwm, tmo_ms);
+ if (ret)
+ goto stop;
+
+ raw_prd = priv->capture[0];
+ raw_dty = priv->capture[1];
+
+ if (raw_dty >= (raw_prd >> icpsc)) {
+ /*
+ * We may fall here using input prescaler, when input
+ * capture starts on high side (before falling edge).
+ * Example with icpsc to capture on each 4 events:
+ *
+ * start 1st capture 2nd capture
+ * v v v
+ * ___ _____ _____ _____ _____ ____
+ * TI1..4 |__| |__| |__| |__| |__|
+ * v v . . . . . v v
+ * icpsc1/3: . 0 . 1 . 2 . 3 . 0
+ * icpsc2/4: 0 1 2 3 0
+ * v v v v
+ * CCR1/3 ......t0..............................t2
+ * CCR2/4 ..t1..............................t1'...
+ * . . .
+ * Capture0: .<----------------------------->.
+ * Capture1: .<-------------------------->. .
+ * . . .
+ * Period: .<------> . .
+ * Low side: .<>.
+ *
+ * Result:
+ * - Period = (t2 - t0) / icpsc
+ * - Duty = Period - Low side = Period - (Capture0 - Capture1)
+ */
+ raw_dty = priv->capture[0] - priv->capture[1];
+ raw_dty = (raw_prd >> icpsc) - raw_dty;
+ }
+
+done:
+ prd = (unsigned long long)raw_prd * (psc + 1) * NSEC_PER_SEC;
+ result->period = DIV_ROUND_UP_ULL(prd, rate << icpsc);
dty = (unsigned long long)raw_dty * (psc + 1) * NSEC_PER_SEC;
result->duty_cycle = DIV_ROUND_UP_ULL(dty, rate);
stop:
diff --git a/include/linux/mfd/stm32-timers.h b/include/linux/mfd/stm32-timers.h
index 219eccc..b67cb28 100644
--- a/include/linux/mfd/stm32-timers.h
+++ b/include/linux/mfd/stm32-timers.h
@@ -77,6 +77,7 @@
#define TIM_BDTR_BK2P BIT(25) /* Break 2 input polarity */

#define MAX_TIM_PSC 0xFFFF
+#define MAX_TIM_ICPSC 0x3
#define TIM_CR2_MMS_SHIFT 4
#define TIM_CR2_MMS2_SHIFT 20
#define TIM_SMCR_TS_SHIFT 4
--
1.9.1

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