Re: Thinkpad T420 and single/dual channel lvds

[Takashi Iwai <tiwai@suse.de>]

At Sun, 18 Mar 2012 19:01:21 +0100,
Andreas Heider wrote:
> 
> Am 18.03.2012 um 18:50 schrieb Daniel Vetter:
> > On Fri, Mar 16, 2012 at 09:29:24PM +0100, Takashi Iwai wrote:
> >> At Fri, 16 Mar 2012 15:55:52 -0400,
> >> Adam Jackson wrote:
> >>> 
> >>> On 3/15/12 10:42 AM, Takashi Iwai wrote:
> >>> 
> >>>> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> >>>> index f851db7..314af26 100644
> >>>> --- a/drivers/gpu/drm/i915/intel_display.c
> >>>> +++ b/drivers/gpu/drm/i915/intel_display.c
> >>>> @@ -356,6 +356,23 @@ static const intel_limit_t intel_limits_ironlake_display_port = {
> >>>>  	.find_pll = intel_find_pll_ironlake_dp,
> >>>>  };
> >>>> 
> >>>> +static bool is_dual_link_lvds(struct drm_i915_private *dev_priv,
> >>>> +			      unsigned int reg)
> >>>> +{
> >>>> +	/* BIOS should set the proper LVDS register value at boot, but
> >>>> +	 * in reality, it doesn't set the value when the lid is closed;
> >>>> +	 * thus when a machine is booted with the lid closed, the LVDS
> >>>> +	 * reg value can't be trusted.  So we need to check "the value
> >>>> +	 * to be set" in VBT at first.
> >>>> +	 */
> >>>> +	if ((dev_priv->bios_lvds_val & LVDS_CLKB_POWER_MASK) ==
> >>>> +	    LVDS_CLKB_POWER_UP)
> >>>> +		return true;
> >>> 
> >>> Would slightly prefer if this was more like:
> >>> 
> >>>     if (dev_priv->bios_lvds_val)
> >>>             return !!(dev_priv->bios_lvds_val & LVDS_CLKB_POWER_MASK);
> >>> 
> >>> Since that way it eliminates some useless register reads in the normal 
> >>> case even for single-link.  Not going to insist on it though.
> >> 
> >> Sounds reasonable, yes.
> >> 
> >> Also, it'd be good to have a module option to override the lvds
> >> channel setup, e.g. lvds_channel=0 for probing BIOS like this,
> >> lvds_channel=1 and 2 are to set the single and dual-channel mode
> >> forcibly, respectively.  If you guys think it's worth, I'll write an
> >> additional patch after fixing this as suggested.
> > 
> > I think we can wait with adding debug options until this blows up. And if
> > there are indeed broken bios out there, we need a full quirk table anyway.
> > 
> > I'll wait for the new patch with Adam's suggestion for merging into -next.
> > -Daniel
> 
> Hi,
> 
> I tried this patch on a Macbook Pro 6,2 and indeed still need to manually set lvds_channel=2. 

OK, that's expected.  Basically my patch would fix only the case where
the default BIOS works but doesn't work in some exceptional case (e.g.
boot with the closed lid).

> It'd be great if this patch with the override parameter or some other way to fix it would end up in mainline.

I think the module option would be good.  It's needed to give an easy
way for testing.  Then we can add the entries in some quirk tables.


thanks,

Takashi