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 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.

-- 
Regards,

Laurent Pinchart