Re: Thinkpad T420 and single/dual channel lvds

[Takashi Iwai <tiwai@suse.de>]

At Sun, 18 Mar 2012 18:50:31 +0100,
Daniel Vetter wrote:
> 
> 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.

The module option is often the easiest way to check, so I believe
it's a good thing.  It won't break the current behavior, at least.

> I'll wait for the new patch with Adam's suggestion for merging into -next.

I'll respin the patches tomorrow with people's suggestions.


thanks,

Takashi