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