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

[Geert Uytterhoeven]

Hi Laurent,

On Wed, Apr 5, 2017 at 10:45 AM, Laurent Pinchart
<laurent.pinchart at ideasonboard.com> wrote:
> 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 ?

That's fine for me (CC Simon). Thx!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds