Patchwork [v2] pwm: i.MX: Avoid sample fifo overflow for i.MX pwm version2

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Submitter Liu Ying
Date April 10, 2014, 9:23 a.m.
Message ID <1397121829-19331-1-git-send-email-Ying.Liu@freescale.com>
Download mbox | patch
Permalink /patch/338072/
State New
Headers show

Comments

Liu Ying - April 10, 2014, 9:23 a.m.
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 <s.hauer@pengutronix.de>
Cc: Shawn Guo <shawn.guo@freescale.com>
Cc: Lothar Waßmann <LW@KARO-electronics.de>
Cc: linux-pwm@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
---
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(+)
Shawn Guo - April 13, 2014, 7:46 a.m.
On Thu, Apr 10, 2014 at 05:23:49PM +0800, Liu Ying wrote:
> 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 <s.hauer@pengutronix.de>
> Cc: Shawn Guo <shawn.guo@freescale.com>

Acked-by: Shawn Guo <shawn.guo@freescale.com>

> Cc: Lothar Waßmann <LW@KARO-electronics.de>
> Cc: linux-pwm@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Liu Ying - April 22, 2014, 8:48 a.m.
Hi Thierry,

On 04/13/2014 03:46 PM, Shawn Guo wrote:
> On Thu, Apr 10, 2014 at 05:23:49PM +0800, Liu Ying wrote:
>> 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 <s.hauer@pengutronix.de>
>> Cc: Shawn Guo <shawn.guo@freescale.com>
> 
> Acked-by: Shawn Guo <shawn.guo@freescale.com>
> 
>> Cc: Lothar Waßmann <LW@KARO-electronics.de>
>> Cc: linux-pwm@vger.kernel.org
>> Cc: linux-arm-kernel@lists.infradead.org
>> Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
> 

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

Thanks.

Patch

diff --git a/drivers/pwm/pwm-imx.c b/drivers/pwm/pwm-imx.c
index cc47733..8929b0d 100644
--- a/drivers/pwm/pwm-imx.c
+++ b/drivers/pwm/pwm-imx.c
@@ -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);