Re: [PATCH] drm/i915: Implement pipe CSC based limited range RGB output

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Jan 18, 2013 at 06:37:09PM +0100, Daniel Vetter wrote:
> On Fri, Jan 18, 2013 at 6:11 PM,  <ville.syrjala@linux.intel.com> wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >
> > HSW no longer has the PIPECONF bit for limited range RGB output.
> > Instead the pipe CSC unit must be used to perform that task.
> >
> > The CSC pre offset are set to 0, since the incoming data is full
> > [0:255] range RGB, the coefficients are programmed to compress the
> > data into [0:219] range, and then we use either the CSC_MODE black
> > screen offset bit, or the CSC post offsets to shift the data to
> > the correct [16:235] range.
> >
> > Also have to change the confiuration of all planes so that the
> > data is sent through the pipe CSC unit. For simplicity send the
> > plane data through the pipe CSC unit always, and in case full
> > range output is requested, the pipe CSC unit is set up with an
> > identity transform to pass the plane data through unchanged.
> >
> > I've been told by some hardware people that the use of the pipe
> > CSC unit shouldn't result in any measurable increase in power
> > consumption numbers.
> >
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> > Note that I haven't actually tested this on HSW. I did test the earlier
> > prototype version on ILK and IVB. The pipe CSC unit on ILK isn't programmed
> > in quite the same as on HSW, but the IVB unit _should_ be identical to HSW.
> >
> > The main risk involves the coefficient registers. If the channel mapping
> > changed for some reason, we could get swapped channels. For some reason
> > reality and documenation didn't seem to agree how the channels are mapped
> > even on ILK and IVB. So I'd like someone to try this out on HSW to make
> > sure the output still looks correct.
> 
> Two questions from the clueless:
> - Since there's some question whether the CSC blows through additional
> power: Can't we just enable it only when we need it? It looks like
> updating the broadcast property requires a full modeset anyway, so we
> don't gain anything by keeping the csc on when not required.

BTW I did a few quick tests on my ILK laptop. There is no difference
in ACPI power numbers from pipe CSC usage, so it seems any increase
in power consumption is so small that ACPI can't measure it. So I'd
say there's no point in worrying about it. Well, assuming the hardware
didn't change too much for HSW.

-- 
Ville Syrjälä
Intel OTC