drm/i915: Fix DPMS and suspend interaction for intel_panel.c

[Jesse Barnes]

On Fri, 11 Mar 2011 02:35:45 +0100 (CET)
"Indan Zupancic" <indan at nul.nu> wrote:

> drm/i915: Fix DPMS and suspend interaction for intel_panel.c
> 
> When suspending intel_panel_disable_backlight() is never called,
> but intel_panel_enable_backlight() is called at resume. With the
> effect that if the brightness was ever changed after screen
> blanking, the wrong brightness gets restored at resume time.
> 
> Nothing guarantees that those calls will be balanced, so having
> backlight_enabled makes no sense, as the real state can change
> without the panel code noticing. So keep things as stateless as
> possible.
> 
> Signed-off-by: Indan Zupancic <indan at nul.nu>

Chris is right that we don't always control the backlight brightness
directly, so we'll want a more complete solution at any rate.

I don't think this one is urgent enough to send upstream now, and it
would be good to make a couple of other fixes as well, while you're
fixing things up. :)  Comments below.

> -/* i915_suspend.c */
> -extern int i915_save_state(struct drm_device *dev);
> -extern int i915_restore_state(struct drm_device *dev);
> -

Looks like a spurious cleanup hunk, should be a separate hunk (possibly
along with other save/restore state cleanups, like removing all the
ILK+ code that probably won't work well :).

> -void intel_panel_setup_backlight(struct drm_device *dev)
> -{
> -	struct drm_i915_private *dev_priv = dev->dev_private;
> -
> -	dev_priv->backlight_level = intel_panel_get_backlight(dev);
> -	dev_priv->backlight_enabled = dev_priv->backlight_level != 0;
>  }

This is getting pretty messy.  Your patch is a step in the right
direction, but I think we may as well go further:
  - suspend/resume of backlight state belongs in the backlight class
    driver
  - that driver should call into the registered i915 backlight handler
    if i915 is controlling it natively (PWM native, combo, legacy)
  - we need a backlight driver struct with function pointers for the
    various calls, so we can split out the PCH functions from the rest;
    might be good to separate native, combo, and legacy that way as
    well; even if results in some code duplication it might make each
    easier to read and less likely to break others when we make changes

Any thoughts about that?  I think Chris and Matthew have been talking
about it as well, so I'd be curious what our backlight final solution
ought to be.

Thanks,
Jesse