Re: Thinkpad T420 and single/dual channel lvds

[Takashi Iwai <tiwai@suse.de>]

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.

> Looks really good otherwise, thanks!
> 
> Reviewed-by: Adam Jackson <ajax@redhat.com>


Thanks!

Takashi