[Intel-gfx] [PATCH 1/2] drm/dp/i915: Fix DP link rate math

[Pandiyan, Dhinakaran]

On Fri, 2016-11-11 at 15:39 +0200, Ville Syrjälä wrote:
> On Wed, Nov 09, 2016 at 09:32:29PM -0800, Dhinakaran Pandiyan wrote:
> > We store DP link rates as link clock frequencies in kHz, just like all
> > other clock values. But, DP link rates in the DP Spec are expressed in
> > Gbps/lane, which seems to have led to some confusion.
> > 
> > E.g., for HBR2
> > Max. data rate = 5.4 Gbps/lane x 4 lane x 8/10 x 1/8 = 2160000 kBps
> > where, 8/10 is for channel encoding and 1/8 is for bit to Byte conversion
> > 
> > Using link clock frequency, like we do
> > Max. data rate = 540000 kHz * 4 lanes = 2160000 kSymbols/s
> > Because, each symbol has 8 bit of data, this is 2160000 kBps
> > and there is no need to account for channel encoding here.
> > 
> > But, currently we do 540000 kHz * 4 lanes * (8/10) = 1728000 kBps
> > 
> > Similarly, while computing the required link bandwidth for a mode,
> > there is a mysterious 1/10 term.
> > This should simply be pixel_clock kHz * bpp * 1/8  to give the final
> > result in kBps
> > 
> > Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 28 +++++++++-------------------
> >  1 file changed, 9 insertions(+), 19 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index 8f313c1..7a9e122 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -161,33 +161,23 @@ static u8 intel_dp_max_lane_count(struct intel_dp *intel_dp)
> >  	return min(source_max, sink_max);
> >  }
> >  
> > -/*
> > - * The units on the numbers in the next two are... bizarre.  Examples will
> > - * make it clearer; this one parallels an example in the eDP spec.
> > - *
> > - * intel_dp_max_data_rate for one lane of 2.7GHz evaluates as:
> > - *
> > - *     270000 * 1 * 8 / 10 == 216000
> > - *
> > - * The actual data capacity of that configuration is 2.16Gbit/s, so the
> > - * units are decakilobits.  ->clock in a drm_display_mode is in kilohertz -
> > - * or equivalently, kilopixels per second - so for 1680x1050R it'd be
> > - * 119000.  At 18bpp that's 2142000 kilobits per second.
> > - *
> > - * Thus the strange-looking division by 10 in intel_dp_link_required, to
> > - * get the result in decakilobits instead of kilobits.
> > - */
> > -
> >  static int
> >  intel_dp_link_required(int pixel_clock, int bpp)
> >  {
> > -	return (pixel_clock * bpp + 9) / 10;
> > +	/* pixel_clock is in kHz, divide bpp by 8 to return the value in kBps*/
> 
> Probably best not to mix in the kBps unit here and instead just talk
> in terms of the symbol clock.
> 
> > +	return (pixel_clock * bpp + 7) / 8;
> 
> DIV_ROUND_UP()
> 
> >  }
> >  
> >  static int
> >  intel_dp_max_data_rate(int max_link_clock, int max_lanes)
> >  {
> > -	return (max_link_clock * max_lanes * 8) / 10;
> > +	/* max_link_clock is the link symbol clock (LS_Clk) in kHz and not the
> > +	 * link rate that is generally expressed in Gbps. Since, 8 bits data is
> > +	 * transmitted every LS_Clk per lane, there is no need to account for
> > +	 * the channel encoding that is done in the PHY layer here.
> > +	 */
> > +
> > +	return (max_link_clock * max_lanes);
> 
> Useless parens.
> 
> 

Thanks for the review, will fix these two lines.

-DK
> >  }
> >  
> >  static int
> > -- 
> > 2.7.4
>