On Wed, 17 Feb 2016 18:30:12 -0600
Adam Ford <aford173@xxxxxxxxx> wrote:
> The backlight pin is shared with Timer 10 PWM. This patch allows the
> pwm_bl driver to enable the pwm run by this timer to dim the backlight.
>
> Signed-off-by: Adam Ford <aford173@xxxxxxxxx>
> ---
[...]
> bl: backlight {
> - compatible = "gpio-backlight";
> + compatible = "pwm-backlight";
> pinctrl-names = "default";
> pinctrl-0 = <&backlight_pins>;
> -
> - gpios = <&gpio5 26 GPIO_ACTIVE_HIGH>, /* gpio_154 */
> - <&gpio2 24 GPIO_ACTIVE_HIGH>; /* gpio_56 */
> - default-on;
> + pwms = <&pwm10 0 5000000 0>;
> + brightness-levels = <0 10 20 30 40 50 60 70 80 90 100>;
> + default-brightness-level = <7>;
> + enable-gpios = <&gpio5 26 GPIO_ACTIVE_HIGH>; /* gpio_154 */
> };
[...]
One thing to note: omap-dmtimer-pwm can't do a perfect 0 or 100% duty cycle.
I think with the code in linux-next right now it will just configure the
DMTIMER in a way that the TRM says is illegal, and with my [PATCH 2/4] it
will set the lowest/highest possible duty-cycle (1-fclk cycle per period).
Assuming you're using the 32768Hz fclk for timer10, that would mean the
lowest effective duty cycle would be about 0.6%, and the highest 99.4%. If
you're using a faster fclk, then the error would be proportionally less.
Just wanted to mention that in case it's an issue for your backlight.
FYI, I've seen other PWM drivers use a trick for the 0 and 100% cases where
they actually force the largest possible period. This then dilutes the
unwanted 1-cycle pulse as much as possible. I see no reason that couldn't
be done for omap-dmtimer-pwm as well.
Looking at the TRM, it may also be possible to turn off the trigger, and
use the TCLR.SCPWM bit to force a perfect 0 or 100% duty cycle.
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