linux-kernel - Re: [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM frequency to 83Hz

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Date:   Mon, 12 Sep 2016 10:41:55 -0400
From:   David Rivshin <drivshin@...rd.com>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     Matthijs van Duin <matthijsvanduin@...il.com>,
        "H. Nikolaus Schaller" <hns@...delico.com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>, marek@...delico.com,
        linux-omap@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, letux-kernel@...nphoenux.org
Subject: Re: [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM
 frequency to 83Hz

On Sat, 10 Sep 2016 15:48:28 +0200
Neil Armstrong <narmstrong@...libre.com> wrote:

> Le 10/09/2016 05:17, Matthijs van Duin a écrit :
> > On Mon, Sep 05, 2016 at 11:16:38AM +0200, H. Nikolaus Schaller wrote:  
> >> This helps to get 100% intensity closer to "always on".
> >>
> >> It compensates for an effect of dmtimer which at 100% still emits short
> >> "off" impulses and the startup-time of the DC/DC converter makes
> >> backlight intensity not reach full scale. The lower the PWM frequency
> >> is, the smaller is this effect.  
> > 
> > Sounds to me like you're working around something that should be fixed
> > in the pwm-omap-dmtimer driver instead?
> > 
> > Looking at the (baremetal) dmtimer pwm code I wrote ages ago, which
> > supports fully off to fully on, I do seem to be handling both endpoints
> > in a special way.  A rough conversion of my code into C:
> > 
> > // period in timer cycles
> > void pwm_init( volatile struct dmtimer *timer, u32 period, bool invert )
> > {
> > 	assert( period >= 2 );
> > 	timer->if_ctrl = 2;  // reset timer, configure as non-posted
> > 	timer->reload = -period;
> > 	timer->trigger = 0;
> > 	timer->config = 0x1043 | invert << 7;  // pwm initially disabled
> > }
> > 
> > // value in timer cycles, 0 <= value <= period
> > void pwm_set( volatile struct dmtimer *timer, u32 value )
> > {
> > 	if( value == 0 ) {
> > 		timer->config &= ~0x800;  // disable pwm
> > 		return;
> > 	}
> > 	u32 period = -timer->reload;
> > 	if( value >= period )
> > 		timer->match = 0;
> > 	else
> > 		timer->match = value - period - 1;
> > 	timer->config |= 0x800;  // enable pwm
> > }
> > 
> > At the time I used a scope to check the exact behaviour of dmtimer pwm
> > on a dm814x.  My notes mention (when pwm enabled):
> > 	match < reload	output on continuous
> > 	match == reload	output on 1 cycle, off period-1 cycles
> > 	match == -2	output on period-1 cycles, off 1 cycle
> > 	match == -1	output freezes
> > 
> > Hope this helps  
> 
> Hi,
> 
> I think these corner cases should definitely be handled in the dmtimer driver.

Do you mean to modify the dmtimer driver itself, or the pwm-omap-dmtimer 
driver?

IIRC from the last time I was in the pwm-omap-dmtimer driver, it seemed to 
me that the 0% and 100% cases could/should be handled as simple special 
cases there. I think the dmtimer driver itself has the necessary API to the 
HW, but I'd need to re-familiarize myself with it to remember the details 
of what I was thinking. 

Actually, I did mention some thoughts on this a previous thread where
Adam Ford was using pwm-omap-dmtimer for a backlight:
 http://www.spinics.net/lists/linux-omap/msg126006.html
So it may be as simple as using PWM_OMAP_DMTIMER_TRIGGER_NONE and passing 
def_on according to whether 0 or 100% duty were requested (and polarity).


> 
> I'll try to post a fix to handle these, thanks for the original code dump.
> 
> > 
> > Matthijs
> >   
> 
> Neil
> --
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