[Intel-gfx] [PATCH 6/7] drm/i915/cnl: Write dco_fraction calculation as spec.

[Rodrigo Vivi]

On Tue, Nov 14, 2017 at 08:22:27PM +0000, Ville Syrjälä wrote:
> On Tue, Nov 14, 2017 at 11:47:58AM -0800, Rodrigo Vivi wrote:
> > I confess I never fully understood that previous calculation,
> > so maybe this is cannot be called a "fix". But let's follow
> > the math that is written on Spec so we have get more confident
> > this is what hardware expect.
> > 
> > Cc: Mika Kahola <mika.kahola at intel.com>
> > Cc: Manasi Navare <manasi.d.navare at intel.com>
> > Cc: James Ausmus <james.ausmus at intel.com>
> > Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dpll_mgr.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dpll_mgr.c b/drivers/gpu/drm/i915/intel_dpll_mgr.c
> > index bd608f7f2399..ee690d2f6e54 100644
> > --- a/drivers/gpu/drm/i915/intel_dpll_mgr.c
> > +++ b/drivers/gpu/drm/i915/intel_dpll_mgr.c
> > @@ -2190,8 +2190,8 @@ static void cnl_wrpll_params_populate(struct skl_wrpll_params *params,
> >  	params->qdiv_mode = (qdiv == 1) ? 0 : 1;
> >  
> >  	params->dco_integer = div_u64(dco_freq, ref_freq);
> > -	params->dco_fraction = div_u64((div_u64((uint64_t)dco_freq<<15, (uint64_t)ref_freq) -
> > -					((uint64_t)params->dco_integer<<15)) * 0x8000, 0x8000);
> > +	params->dco_fraction = (DIV_ROUND_UP_ULL(dco_freq, ref_freq) -
> > +				params->dco_integer) * (1 << 15);
> 
> Is dco_freq in khz here? Or hz?
> 
> If khz, the following should work... I think:

khz

> 
> unsigned int dco = div_u64((u64)dco_freq << 15, ref_freq * 1000);

why to convert ref_freq? isn't this in khz as well already?

> dco_int = dco >> 15;
> dco_frac = dco & 0x7fff;

wow! my first thought was that was more complicated, but eureka,
this should be the right way to go...

Clearly better than spec... like that maximum in the other patch ;)

> 
> >  }
> >  
> >  static bool
> > -- 
> > 2.13.6
> > 
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> 
> -- 
> Ville Syrjälä
> Intel OTC
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