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

[Shawn Guo]

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 at pengutronix.de>
> Cc: Shawn Guo <shawn.guo at freescale.com>

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

> Cc: Lothar Waßmann <LW at KARO-electronics.de>
> Cc: linux-pwm at vger.kernel.org
> Cc: linux-arm-kernel at lists.infradead.org
> Signed-off-by: Liu Ying <Ying.Liu at freescale.com>