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From: Fabrice Gasnier <fabrice.gasnier@st.com>
To: <lee.jones@linaro.org>, <thierry.reding@gmail.com>,
	<robh+dt@kernel.org>, <alexandre.torgue@st.com>,
	<benjamin.gaignard@linaro.org>
Cc: <mark.rutland@arm.com>, <linux@armlinux.org.uk>,
	<mcoquelin.stm32@gmail.com>, <fabrice.gasnier@st.com>,
	<benjamin.gaignard@st.com>, <devicetree@vger.kernel.org>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, <linux-pwm@vger.kernel.org>
Subject: [PATCH 6/8] pwm: stm32: improve capture by tuning counter prescaler
Date: Tue, 16 Jan 2018 13:43:49 +0100	[thread overview]
Message-ID: <1516106631-18722-7-git-send-email-fabrice.gasnier@st.com> (raw)
In-Reply-To: <1516106631-18722-1-git-send-email-fabrice.gasnier@st.com>

Currently, capture is based on timeout window to configure prescaler.
PWM capture framework provides 1s window at the time of writing.

There's place for improvement, after input signal has been captured once:
- Finer tune counter clock prescaler, by using 1st capture result (with
arbitrary margin).
- Do a 2nd capture, with scaled capture window.
This increases accuracy, especially at high rates.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
---
 drivers/pwm/pwm-stm32.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c
index b9c7e878..c890404 100644
--- a/drivers/pwm/pwm-stm32.c
+++ b/drivers/pwm/pwm-stm32.c
@@ -257,7 +257,7 @@ 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;
+	unsigned int psc = 0, scale;
 	u32 raw_prd, raw_dty;
 	int ret = 0;
 
@@ -311,6 +311,30 @@ static int stm32_pwm_capture(struct pwm_chip *chip, struct pwm_device *pwm,
 	raw_prd = priv->capture[0];
 	raw_dty = priv->capture[1];
 
+	/*
+	 * Got a capture. Try to improve accuracy at high rates:
+	 * - decrease counter clock prescaler, scale up to max rate.
+	 */
+	if (raw_prd) {
+		u32 max_arr = priv->max_arr - 0x1000; /* arbitrary margin */
+
+		scale = max_arr / min(max_arr, raw_prd);
+	} else {
+		scale = priv->max_arr; /* bellow resolution, use max scale */
+	}
+
+	if (psc && scale > 1) {
+		/* 2nd measure with new scale */
+		regmap_write(priv->regmap, TIM_PSC, psc / scale);
+		ret = stm32_pwm_do_capture(priv, pwm, tmo_ms);
+		if (ret)
+			goto stop;
+
+		psc /= scale;
+		raw_prd = priv->capture[0];
+		raw_dty = priv->capture[1];
+	}
+
 	prd = (unsigned long long)raw_prd * (psc + 1) * NSEC_PER_SEC;
 	result->period = DIV_ROUND_UP_ULL(prd, rate);
 	dty = (unsigned long long)raw_dty * (psc + 1) * NSEC_PER_SEC;
-- 
1.9.1

  parent reply	other threads:[~2018-01-16 12:45 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-16 12:43 [PATCH 0/8] Add support for PWM input capture on STM32 Fabrice Gasnier
2018-01-16 12:43 ` [PATCH 1/8] pwm: stm32: fix, remove unused struct device Fabrice Gasnier
2018-01-16 12:43 ` [PATCH 2/8] pwm: stm32: protect common prescaler for all channels Fabrice Gasnier
2018-01-16 12:43 ` [PATCH 3/8] dt-bindings: mfd: stm32-timers: add support for dmas Fabrice Gasnier
2018-01-16 12:43 ` [PATCH 4/8] " Fabrice Gasnier
2018-01-23 13:32   ` Lee Jones
2018-01-23 13:57     ` Fabrice Gasnier
2018-01-23 15:30       ` Lee Jones
2018-01-23 15:52         ` Fabrice Gasnier
2018-01-23 16:41           ` Lee Jones
2018-01-24  8:40             ` Fabrice Gasnier
2018-01-24 14:56               ` Lee Jones
2018-01-24 15:30                 ` Fabrice Gasnier
2018-01-24 15:43                   ` cas
2018-01-16 12:43 ` [PATCH 5/8] pwm: stm32: add capture support Fabrice Gasnier
2018-01-16 12:43 ` Fabrice Gasnier [this message]
2018-01-16 12:43 ` [PATCH 7/8] pwm: stm32: use input prescaler to improve period capture Fabrice Gasnier
2018-01-16 12:43 ` [PATCH 8/8] ARM: dts: stm32: Enable pwm3 input capture on stm32f429i-eval Fabrice Gasnier

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