Re: [PATCH] fix backlight brightness on intel LVDS panel after reopening lid

[Takashi Iwai <tiwai@suse.de>]

At Thu, 10 Mar 2011 08:49:37 +0100,
Takashi Iwai wrote:
> 
> At Thu, 10 Mar 2011 06:50:09 +0100 (CET),
> Indan Zupancic wrote:
> > 
> > Hello,
> > 
> > On Fri, March 4, 2011 19:47, Linus Torvalds wrote:
> > > Alex, can you confirm that the revert of 951f3512dba5 plus the
> > > one-liner patch from Takashi that Indan quoted also works for you?
> > >
> > >               Linus
> > >
> > > On Thu, Mar 3, 2011 at 10:53 PM, Indan Zupancic <indan@nul.nu> wrote:
> > >>
> > >> So please revert my patch and apply Takashi Iwai's, which fixes the
> > >> most immediate bug without changing anything else. This should go
> > >> in stable too.
> > >
> > 
> > I found another backlight bug:
> > 
> > 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.
> > 
> > This explains the weird behaviour I've seen. I didn't see it with
> > combination mode, because then the brightness is always the same
> > (zero or the maximum, the BIOS only uses LBPC on my system.) I'll
> > send a patch in a moment.
> > 
> > Alternative for reverting the combination mode removal (I can also
> > redo the patch against the revert and Takashi's patch, if that's
> > preferred):
> > 
> > --
> > 
> > drm/i915: Do handle backlight combination mode specially
> > 
> > Add back the combination mode check, but with slightly cleaner code
> > and the weirdness removed: No val >>= 1, but also no val &= ~1. The
> > old code probably confused bit 0 with BLM_LEGACY_MODE, which is bit 16.
> > The other change is clearer calculations: Just check for zero level
> > explicitly instead of avoiding the divide-by-zero.
> > 
> > Signed-off-by: Indan Zupancic <indan@nul.nu>
> > 
> > ---
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> > index d860abe..b05631a 100644
> > --- a/drivers/gpu/drm/i915/intel_panel.c
> > +++ b/drivers/gpu/drm/i915/intel_panel.c
> > @@ -30,6 +30,10 @@
> > 
> >  #include "intel_drv.h"
> > 
> > +#define PCI_LBPC 0xf4 /* legacy/combination backlight modes */
> > +#define BLM_COMBINATION_MODE (1 << 30)
> > +#define BLM_LEGACY_MODE (1 << 16)
> > +
> >  void
> >  intel_fixed_panel_mode(struct drm_display_mode *fixed_mode,
> >  		       struct drm_display_mode *adjusted_mode)
> > @@ -110,6 +114,22 @@ done:
> >  	dev_priv->pch_pf_size = (width << 16) | height;
> >  }
> > 
> > +/*
> > + * What about gen 3? If there are no gen 3 systems with ASLE,
> > + * then it doesn't matter, as we don't need to change the
> > + * brightness. But then the gen 2 check can be removed too.
> > + */
> > +static int is_backlight_combination_mode(struct drm_device *dev)
> > +{
> > +	struct drm_i915_private *dev_priv = dev->dev_private;
> > +
> > +	if (INTEL_INFO(dev)->gen >= 4)
> > +		return I915_READ(BLC_PWM_CTL2) & BLM_COMBINATION_MODE;
> > +	if (IS_GEN2(dev))
> > +		return I915_READ(BLC_PWM_CTL) & BLM_LEGACY_MODE;
> > +	return 0;
> > +}
> > +
> >  static u32 i915_read_blc_pwm_ctl(struct drm_i915_private *dev_priv)
> >  {
> >  	u32 val;
> > @@ -163,9 +183,12 @@ u32 intel_panel_get_max_backlight(struct drm_device *dev)
> >  			max >>= 17;
> >  		} else {
> >  			max >>= 16;
> > +			/* Ignore BLM_LEGACY_MODE bit */
> >  			if (INTEL_INFO(dev)->gen < 4)
> >  				max &= ~1;
> >  		}
> > +		if (is_backlight_combination_mode(dev))
> > +			max *= 0xff;
> >  	}
> > 
> >  	DRM_DEBUG_DRIVER("max backlight PWM = %d\n", max);
> > @@ -183,6 +206,12 @@ u32 intel_panel_get_backlight(struct drm_device *dev)
> >  		val = I915_READ(BLC_PWM_CTL) & BACKLIGHT_DUTY_CYCLE_MASK;
> >  		if (IS_PINEVIEW(dev))
> >  			val >>= 1;
> > +		if (is_backlight_combination_mode(dev)){
> > +			u8 lbpc;
> > +
> > +			pci_read_config_byte(dev->pdev, PCI_LBPC, &lbpc);
> > +			val *= lbpc;
> > +		}
> >  	}
> > 
> >  	DRM_DEBUG_DRIVER("get backlight PWM = %d\n", val);
> > @@ -205,6 +234,15 @@ void intel_panel_set_backlight(struct drm_device *dev, u32 level)
> > 
> >  	if (HAS_PCH_SPLIT(dev))
> >  		return intel_pch_panel_set_backlight(dev, level);
> > +
> > +	if (level && is_backlight_combination_mode(dev)){
> > +		u32 max = intel_panel_get_max_backlight(dev);
> > +		u8 lpbc;
> > +
> > +		lpbc = level * 0xff / max;
> > +		level /= lpbc;
> 
> Hmm, I don't think this calculation is correct.  This would result
> in level of opregion over its limit.  For example, assume the level
> max = 100, so total max = 25500.  Passing level=150 here will be:
> 
> 	lbpc = 150 * 0xff / 25500 = 1.5 = 1
> 	level = 150 / 1 = 150, which is over limit.
> 
> More worse, lbpc can be zero when level is below 100 in the case
> above...

That is, Chris' original code in that portion was correct:

	if (is_backlight_combination_mode(dev)){
		u32 max = intel_panel_get_max_backlight(dev);
		u8 lpbc;

		lpbc = level * 0xfe / max + 1;
		level /= lpbc;
		pci_write_config_byte(dev->pdev, PCI_LBPC, lpbc);
	}

This will fit within the right range.
Though, changing like below will give a bit better calculation,
closer to the real level.

		lpbc = level * 0xfe / max + 1;
		level = (level + lpbc / 2) / lpbc;


Takashi