Re: [PATCH v2 09/13] arm64: dts: r8a7795: salvator-x: Add panel backlight support

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Geert,

On Monday 21 Nov 2016 11:59:47 Laurent Pinchart wrote:
> On Monday 21 Nov 2016 10:23:46 Geert Uytterhoeven wrote:
> > On Mon, Nov 21, 2016 at 10:19 AM, Laurent Pinchart wrote:
> >> On Monday 21 Nov 2016 09:36:22 Geert Uytterhoeven wrote:
> >>> On Sat, Nov 19, 2016 at 4:28 AM, Laurent Pinchart wrote:
> >>>> The panel backlight is controlled through a GPIO and a PWM channel.
> >>>> 
> >>>> --- a/arch/arm64/boot/dts/renesas/r8a7795-salvator-x.dts
> >>>> +++ b/arch/arm64/boot/dts/renesas/r8a7795-salvator-x.dts
> >>>> @@ -178,6 +178,16 @@
> >>>>                         };
> >>>>                 };
> >>>>         };
> >>>> +
> >>>> +       backlight: backlight {
> >>>> +               compatible = "pwm-backlight";
> >>>> +               pwms = <&pwm1 0 50000>;
> >>>> +
> >>>> +               brightness-levels = <256 128 64 16 8 4 0>;
> >>> 
> >>> Would it make sense to define more and/or linear levels?
> >> 
> >> Possibly, this is pretty arbitrary. Linear levels might not be the best
> >> option given that the human eye doesn't have a linear response to light
> >> power, but we
> > 
> > It not only depends on the human eye, but also on the backlight hardware
> > (is the conversion from voltage (L_VBRT) to light linear?).
> 
> So we need to specify transfer functions in DT ;-)
> 
> >> could certainly have more levels. In that case I'd prefer modifying the
> >> pwm- backlight DT bindings though, and specifying the PWM resolution
> >> instead of discrete levels.
> >> 
> >> Note that the LVDS panel backlight PWM control signal is multiplexed
> >> with the external memory A21 signal on the Salvator-X board, with SW5
> >> selecting which how to route the signal. When using backlight control we
> >> can't access the whole NOR flash anymore, so I'm not sure this patch
> >> should be merged.
> > 
> > That NOR flash is also optional, right?
> > My Ex Memory Connector is not populated.
> 
> That's correct. The Salvator-X DT file in mainline is just an example
> anyway, and we should pick the most useful peripherals for that purpose.

Would you like me to include this in my next Salvator-X DT patch series for 
upstream merge ?

-- 
Regards,

Laurent Pinchart