linux-kernel - Re: [PATCH v5 3/3] pwm: i.MX: Avoid sample FIFO overflow for i.MX PWM version2

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Date:	Mon, 25 Aug 2014 19:59:01 +0800
From:	Shawn Guo <shawn.guo@...escale.com>
To:	Liu Ying <Ying.Liu@...escale.com>
CC:	<linux-pwm@...r.kernel.org>, <thierry.reding@...il.com>,
	<s.hauer@...gutronix.de>, <LW@...O-electronics.de>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3/3] pwm: i.MX: Avoid sample FIFO overflow for i.MX
 PWM version2

On Wed, May 28, 2014 at 06:50:13PM +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 is a 4-word(16bit)
> 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@...il.com>
> Cc: Sascha Hauer <s.hauer@...gutronix.de>
> Cc: Shawn Guo <shawn.guo@...escale.com>
> Cc: Lothar Waßmann <LW@...O-electronics.de>
> Cc: linux-pwm@...r.kernel.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Signed-off-by: Liu Ying <Ying.Liu@...escale.com>

The whole series,

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

Thierry,

Can you pick them up if they look good to you?

Shawn
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