backlight - chicken and egg challenge

Sam Ravnborg sam at
Sat Sep 8 20:17:55 UTC 2018

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?


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