On Sat, 10 Sep 2016 15:48:28 +0200
Neil Armstrong <narmstr...@baylibre.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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