Re: [PATCH 2/3] tegra: pwm-backlight: add tegra pwm-bl driver

[Alex Courbot <acourbot@nvidia.com>]

> I'm confused. Why would you want to call into pwm_bl directly? If we're
> going to split this up into separate platform devices, why not look up a
> given backlight device and use the backlight API on that? The pieces of
> the puzzle are all there: you can use of_find_backlight_by_node() to
> obtain a backlight device from a device tree node, so I'd expect the DT
> to look something like this:
> 
> 	backlight: backlight {
> 		compatible = "pwm-backlight";
> 		...
> 	};

This would still prevent any power control from the backlight driver. I.e. if 
someone sets the brightness to 0 through sysfs, we cannot power the backlight 
off as pwm-backlight cannot control more than the PWM without platform 
callbacks. Backlight could only be powered off as a result of a fb blank event.

> 	panel: panel {
> 		compatible = "...";
> 		...
> 		backlight = <&backlight>;
> 		...
> 	};

So all the power control of both the panel and backlight would be performed 
from this device's driver. How would it plug into tegra-drm? I would see 
tegra_panel as a new member of the tegra_output structure, with one callback 
invoked from tegra_encoder_dpms(). Does that look sane?

> After that you can wire it up with host1x using something like:
> 
> 	host1x {
> 		dc@54200000 {
> 			rgb {
> 				status = "okay";
> 
> 				nvidia,panel = <&panel>;
> 			};
> 		};
> 	};

Indeed. So if we do that, the DRM DPMS functions would take care of the 
panel/backlight powering and the backlight driver will control the PWM after 
this, through the FB notifier. This is a little bit different from the "official" 
power sequence, but I just tested controlling the PWM at the very end of the 
sequence and it works just as well. If you think this looks better I don't 
mind doing it that way, it is actually a good excuse for me to dive into the 
DRM code.

Anyway, this will only be a temporary solution, CDF is the only way to do this 
right.

Alex.