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

[Geert Uytterhoeven]

Hi Laurent,

On Mon, Nov 21, 2016 at 10:19 AM, Laurent Pinchart
<laurent.pinchart at ideasonboard.com> 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?).

> 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.

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