linux-kernel - Re: i915/kms/backlight-combo mode problem (was: Re: Linux 2.6.39-rc5)

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Message-ID: <s5hwricc021.wl%tiwai@suse.de>
Date:	Fri, 29 Apr 2011 21:47:02 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Melchior FRANZ <melchior.franz@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Chris Wilson <chris@...is-wilson.co.uk>,
	dri-devel@...ts.freedesktop.org
Subject: Re: i915/kms/backlight-combo mode problem (was: Re: Linux 2.6.39-rc5)

At Fri, 29 Apr 2011 19:41:53 +0200,
Melchior FRANZ wrote:
> 
> * Takashi Iwai -- Friday 29 April 2011:
> > Melchior FRANZ wrote:
> > > The bug was introduced with commit ba3820ade317ee36e496b9b40d2ec3987dd4aef0
> > > [...] when using KMS my notebook's[2] screen remains dark, because the
> > > backlight isn't turned on.
> 
> > Could you check whether the patch below changes the behavior?
> > If this cures, it means that the backlight-combo mode doesn't work on
> > your machine.
> 
> Yes, that works. (Test was with fafc9929c668f8bae6dd1f109f33a86d2cb3c460,
> which is current HEAD.)

Looking at bugzilla, the problem seems like the case lbpc=0.
What about the patch below instead?


Takashi

---
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index a06ff07..ba60218 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -207,7 +207,8 @@ u32 intel_panel_get_backlight(struct drm_device *dev)
 
 			val &= ~1;
 			pci_read_config_byte(dev->pdev, PCI_LBPC, &lbpc);
-			val *= lbpc;
+			if (lbpc)
+				val *= lbpc;
 		}
 	}
 
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