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

[Takashi Iwai <tiwai@suse.de>]

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...


thanks,

Takashi