Message-ID: <s5hsjuvm8qt.wl%tiwai@suse.de>
Date:	Thu, 10 Mar 2011 13:59:38 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	"Indan Zupancic" <indan@....nu>
Cc:	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Alex Riesen" <raa.lkml@...il.com>,
	"Jesse Barnes" <jbarnes@...tuousgeek.org>,
	"DRI mailing list" <dri-devel@...ts.freedesktop.org>,
	"Chris Wilson" <chris@...is-wilson.co.uk>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Tino Keitel" <tino.keitel@...ei.de>, stable@...nel.org
Subject: Re: [PATCH] fix backlight brightness on intel LVDS panel after           reopening lid

At Thu, 10 Mar 2011 11:06:28 +0100 (CET),
Indan Zupancic wrote:
> 
> On Thu, March 10, 2011 09:25, Takashi Iwai wrote:
> > 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@....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@....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;
> 
> Indeed, though I don't think it makes much difference in practise.
> 
> All in all it seems best to just revert my patch and apply your fix.
> Any "improvements" I may have are either buggy or can be added later.

Agreed.  We should take a safer way.

> Care to make a new patch with the above improvement added? You can
> add my acked-by, for what it's worth.

OK, I'm going to send it now.

> At this point I don't even dare removing that "obviously" bogus
> val &= ~1; I bet it's an undocumented bit having some obscure
> secret function on not well tested systems.

Yes, I also left it...


Takashi
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