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

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Date:   Sat, 10 Sep 2016 09:14:07 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Discussions about the Letux Kernel <letux-kernel@...nphoenux.org>
Cc:     Matthijs van Duin <matthijsvanduin@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, David Rivshin <drivshin@...worx.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Benoît Cousson <bcousson@...libre.com>,
        linux-omap@...r.kernel.org
Subject: Re: [Letux-kernel] [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM frequency to 83Hz

> Am 10.09.2016 um 09:08 schrieb H. Nikolaus Schaller <hns@...delico.com>:
> 
> Hi,
> 
>> Am 10.09.2016 um 05:17 schrieb Matthijs van Duin <matthijsvanduin@...il.com>:
>> 
>> 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
> 
> Yes and no.
> 
> Reducing the PWM frequency is good by itself since it should not be unnecessarily
> fast and helps to make the PWM to "average current" translation more linear.
> 
> The non-linear effect is that the PWM controlled DC/DC converter reacts almost
> immediately to a 1->0 control transition but needs some time (ca. 0.5ms) to recover
> on a 0->1 transition. So if you run PWM @ 500Hz and 100% there is 1ms 1 and 1 ms 0.

was too early in the morning: should be PWM @ 500Hz and 50%.

> But this translates to 1.5 ms no power and 0.5ms power which is 50% of the intended
> current.

At PWM at 100% with the current PWM driver we still get a DC/DC control of 1.95ms 1
and 0.05ms 0 which translates into 0.55ms no power and 1.45 ms power which is only 75%
of the desired maximum.

> 
> This gives some "reduction" factor to all PWM duty cycle values, but the 100%
> case is the most noticeable one.
> 
> If we just fix the PWM generator to output a steady 1 signal at 100%, we have a
> very significant change if we switch to 99%, depending on PWM frequency.
> 
> This effect becomes smaller if the PWM frequency is reduced and 83Hz seems more
> reasonable (although still a little arbitrary) than the current value. (BTW: for
> the Pyra we already use 83Hz).
> 
>> in the pwm-omap-dmtimer driver instead?
> 
> Yes, it probably should be fixed as well but it does not completely solve
> the backlight control issue due to the DC/DC converter's behaviour.
> 
>> 
>> 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
>> 
>> Matthijs
> 
> BR,
> Nikolaus
> 
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