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