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pwm: i.MX: Avoid sample fifo overflow for i.MX pwm version2

PATCH Superseded Liu Ying 2 participants 3 messages Apr 10, 2014 – Apr 22, 2014

Subsystems (2)
Arm i.MX and Layerscape PWM

v2 of 5 Superseded Latest is v5 →

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Liu Ying

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#0 lore raw

PATCH v2 1/1 pwm: i.MX: Avoid sample fifo overflow for i.MX pwm version2

The i.MX pwm version2 is embedded in several i.MX SoCs, such as i.MX27, i.MX51 and i.MX6SL. There are four 16bit sample fifos in this IP, each of which determines the duty period of a PWM waveform in one full cycle. The IP spec mentions that we should not write a fourth sample because the fifo will become full and trigger a fifo write error(FWE) which will prevent the PWM from starting once it is enabled. In order to avoid any sample fifo overflow issue, this patch does software reset to clear all the sample fifos in the very beginning of the pwm configuration function.

The fifo overflow issue can be reproduced by the following commands on the i.MX6SL evk platform, assuming we use pwm2 for the debug LED which is driven by the pin HSIC_STROBE and the maximal brightness is 255.

echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness

echo 255 > /sys/class/leds/user/brightness Here, FWE happens(PWMSR register reads 0x58) and the LED can not be lighten.

Cc Sascha Hauer <[email protected]>
Cc Shawn Guo <[email protected]>
Cc Lothar Waßmann <[email protected]>
Cc [email protected]
Cc [email protected]
Signed-off-by Liu Ying <[email protected]>

---
v1->v2:
* To address Lothar Waßmann's comment, add a timeout mechanism instead of endless polling the SWR bit to be cleared by the hardware.

 drivers/pwm/pwm-imx.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)
drivers/pwm/pwm-imx.c +17 -0
index cc47733..8929b0d 100644
@@ -14,6 +14,7 @@
 #include <linux/slab.h>
 #include <linux/err.h>
 #include <linux/clk.h>
+#include <linux/delay.h>
 #include <linux/io.h>
 #include <linux/pwm.h>
 #include <linux/of.h>

@@ -38,8 +39,11 @@
 #define MX3_PWMCR_DBGEN			(1 << 22)
 #define MX3_PWMCR_CLKSRC_IPG_HIGH (2 << 16)
 #define MX3_PWMCR_CLKSRC_IPG      (1 << 16)
+#define MX3_PWMCR_SWR		  (1 << 3)
 #define MX3_PWMCR_EN              (1 << 0)
 
+#define MX3_PWM_SWR_LOOP	  5
+
 struct imx_chip {
 	struct clk	*clk_per;
 	struct clk	*clk_ipg;

@@ -103,10 +107,23 @@ static int imx_pwm_config_v2(struct pwm_chip *chip,
 		struct pwm_device *pwm, int duty_ns, int period_ns)
 {
 	struct imx_chip *imx = to_imx_chip(chip);
+	struct device *dev = chip->dev;
 	unsigned long long c;
 	unsigned long period_cycles, duty_cycles, prescale;
+	int wait_count = 0;
 	u32 cr;
 
+	/* do software reset in case fifo overflows */
+	writel(MX3_PWMCR_SWR, imx->mmio_base + MX3_PWMCR);
+	do {
+		usleep_range(200, 1000);
+		cr = readl(imx->mmio_base + MX3_PWMCR);
+	} while ((cr & MX3_PWMCR_SWR) &&
+		 (wait_count++ < MX3_PWM_SWR_LOOP));
+
+	if (cr & MX3_PWMCR_SWR)
+		dev_warn(dev, "software reset timeout\n");
+
 	c = clk_get_rate(imx->clk_per);
 	c = c * period_ns;
 	do_div(c, 1000000000);
Signature · 1 line
1.7.9.5
#1 lore raw

PATCH v2 1/1 pwm: i.MX: Avoid sample fifo overflow for i.MX pwm version2

Quoted 19 lines · Liu Ying

The i.MX pwm version2 is embedded in several i.MX SoCs, such as i.MX27, i.MX51 and i.MX6SL. There are four 16bit sample fifos in this IP, each of which determines the duty period of a PWM waveform in one full cycle. The IP spec mentions that we should not write a fourth sample because the fifo will become full and trigger a fifo write error(FWE) which will prevent the PWM from starting once it is enabled. In order to avoid any sample fifo overflow issue, this patch does software reset to clear all the sample fifos in the very beginning of the pwm configuration function.

The fifo overflow issue can be reproduced by the following commands on the i.MX6SL evk platform, assuming we use pwm2 for the debug LED which is driven by the pin HSIC_STROBE and the maximal brightness is 255.

echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness
Quoted 6 lines

echo 255 > /sys/class/leds/user/brightness Here, FWE happens(PWMSR register reads 0x58) and the LED can not be lighten.

Cc Sascha Hauer <[email protected]>
Cc Shawn Guo <[email protected]>
Acked-by Shawn Guo <[email protected]>
Quoted 4 lines
Cc Lothar Wa?mann <[email protected]>
Cc linux-pwm at vger.kernel.org
Cc linux-arm-kernel at lists.infradead.org
Signed-off-by Liu Ying <[email protected]>
author
#2 lore raw

PATCH v2 1/1 pwm: i.MX: Avoid sample fifo overflow for i.MX pwm version2

Hi Thierry,

Quoted 24 lines · Shawn Guo
Quoted 23 lines · Liu Ying

The i.MX pwm version2 is embedded in several i.MX SoCs, such as i.MX27, i.MX51 and i.MX6SL. There are four 16bit sample fifos in this IP, each of which determines the duty period of a PWM waveform in one full cycle. The IP spec mentions that we should not write a fourth sample because the fifo will become full and trigger a fifo write error(FWE) which will prevent the PWM from starting once it is enabled. In order to avoid any sample fifo overflow issue, this patch does software reset to clear all the sample fifos in the very beginning of the pwm configuration function.

The fifo overflow issue can be reproduced by the following commands on the i.MX6SL evk platform, assuming we use pwm2 for the debug LED which is driven by the pin HSIC_STROBE and the maximal brightness is 255.

echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness
echo 0   > /sys/class/leds/user/brightness

echo 255 > /sys/class/leds/user/brightness Here, FWE happens(PWMSR register reads 0x58) and the LED can not be lighten.

Quoted 10 lines
Quoted 2 lines
Cc Sascha Hauer <[email protected]>
Cc Shawn Guo <[email protected]>
Acked-by Shawn Guo <[email protected]>
Quoted 4 lines
Cc Lothar Wa?mann <[email protected]>
Cc linux-pwm at vger.kernel.org
Cc linux-arm-kernel at lists.infradead.org
Signed-off-by Liu Ying <[email protected]>

I've got Shawn's ack on this patch.
If no objections, could you take it into your PWM tree?

Thanks.

Signature · 1 line
Liu Ying