• Liu Ying's avatar
    pwm: imx: Avoid sample FIFO overflow for i.MX PWM version2 · 137fd45f
    Liu Ying authored
    The i.MX PWM version2 is embedded in several i.MX SoCs, such as i.MX27,
    i.MX51 and i.MX6SL.  There is a 4-word (16 bit) sample FIFO in this IP.
    Each FIFO slot 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 triggers 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 clears all sample
    FIFO by doing software reset in the configuration hook when the
    controller is disabled or waits for a full PWM cycle to get a
    relinquished FIFO slot when the controller is enabled and the FIFO is
    fully loaded.
    
    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.
    
    Another way to reproduce the FIFO overflow issue is to run this script:
    
    	while true;
    		do echo 255 > /sys/class/leds/user/brightness;
    	done
    
    Cc: Thierry Reding <thierry.reding@gmail.com>
    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: default avatarLiu Ying <Ying.Liu@freescale.com>
    Acked-by: default avatarShawn Guo <shawn.guo@freescale.com>
    Signed-off-by: default avatarThierry Reding <thierry.reding@gmail.com>
    137fd45f
pwm-imx.c 8.67 KB