backlight - chicken and egg challenge

[Boris Brezillon]

Hi Sam,

On Sat, 8 Sep 2018 22:17:55 +0200
Sam Ravnborg <sam at ravnborg.org> wrote:

> Hi all.
> 
> When working on the DRM driver for Atmel LCDC the first approach
> was to use a MFD driver, that had two sub-drivers:
> - PWM dirver
> - DRM driver
> 
> Feedback was that the PWM feature was too small to warrant a MFD driver.
> (There was no consencus on this, but I anyway went ahead).
> 
> So the new approch is much simpler (from a code point of view):
> DRM Driver that has one sub-driver
> - PWM driver
>   The PWM driver uses registers in the same memory
>   range as the DRM driver, so the two drivers uses
>   the same regmap.
> 
>   The PWM driver is located in pwm/
>   The DRM driver is located in gpu/drm/atmel
>     The DRM driver uses platform_device_register_data() to
>     register the sub-device/driver.
>     I have yet to see it work, but I think this is the right way
>     to do it.
> 
> This all looked fine until reality kicked in.
> 
> 
> There is the following dependency chain (=> depends on):
> DRM Driver => Panel => Backlight => PWM => DRM Driver
> 
> The DRM Driver rely indirectly on the DRM driver, which it not OK.
> 
> So the open question is how to fix this dependency challenge?
> 
> 1) Drop the generic backlight driver and implement all pwm/backlight
>    handling in the driver.
> 2) Re-introduce the MFD driver.
> 3) ?
> 
> Any good ideas?

I keep thinking #2 is the cleanest option. The fact is, the LCDC IP is
actually providing 2 functionalities: a PWM (most of the time driving a
backlight) and a display controller. Really, I don't see why
representing this IP as an MFD is not appropriate, especially since the
HLCDC MFD driver is already in place and can easily be re-used.

What should be adjusted though, is the need for 2 sub-nodes: I think you
can just use a single node and declare #pwm-cells at the top level.

Regards,

Boris

Regards,

Boris