On Fri, Dec 13, 2013 at 01:42:40PM +0100, Lothar Waßmann wrote: > Hi, > > Shawn Guo wrote: > > On Fri, Dec 13, 2013 at 10:54:17AM +0100, Lothar Waßmann wrote: > > > > > + compatible = "pwm-backlight"; > > > > > + pwms = <&pwm2 0 500000>; > > > > > + power-supply = <®_3v3>; > > > > > + brightness-levels = < > > > > > + 100 99 98 97 96 95 94 93 92 91 > > > > > + 90 89 88 87 86 85 84 83 82 81 > > > > > + 80 79 78 77 76 75 74 73 72 71 > > > > > + 70 69 68 67 66 65 64 63 62 61 > > > > > + 60 59 58 57 56 55 54 53 52 51 > > > > > + 50 49 48 47 46 45 44 43 42 41 > > > > > + 40 39 38 37 36 35 34 33 32 31 > > > > > + 30 29 28 27 26 25 24 23 22 21 > > > > > + 20 19 18 17 16 15 14 13 12 11 > > > > > + 10 9 8 7 6 5 4 3 2 1 > > > > > + 0 > > > > > > > > Why is it so unique to start with 100 and end with 0? Does it even > > > > work? Here is what I read from bindings doc. > > > > > > > Yes, it works. The backlight control of the displays we typically use > > > is active low. This is a poor man's way to get an inverted PWM output! > > > (And one in which the number written to the sysfs file corresponds to > > > the actual duty cycle of the PWM output) > > > > I think Linux pwm driver should be improved for this case rather than > > manipulate device tree in a way that violate the defined bindings. > > > I admit I would be happier with a 'polarity' flag like other PWM > drivers provide it. I tried to add polarity support in the generic PWM > layer, so that all HW drivers that do not support inversion in HW can > make use of it, but this gives problems with client drivers that don't > know about the inversion and disable the PWM when the duty cycle is set > to 0. > Maybe I'll find some time to find a proper solution for this issue. Actually the i.MX supports inversion in hardware. It's bit 18 in the PWMCR register. As a sidenote IMO client drivers shouldn't make assumptions about the pwm output when a pwm is disabled anyway. If clients want to have a constant output they should set the duty cycle to 0 or 100%. Sascha -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
