Re: [PATCH] drm/i915/bxt: Broxton doesn't use gen9 scaling for rps frequencies.

[Imre Deak <imre.deak@intel.com>]

On to, 2015-11-12 at 10:14 -0800, Bob Paauwe wrote:
> On Thu, 12 Nov 2015 10:35:00 +0200
> Imre Deak <imre.deak@intel.com> wrote:
> 
> > On Wed, 2015-11-11 at 13:36 -0800, Bob Paauwe wrote:
> > > On Tue, 10 Nov 2015 11:04:22 +0200
> > > Mika Kuoppala <mika.kuoppala@linux.intel.com> wrote:
> > > 
> > > > Bob Paauwe <bob.j.paauwe@intel.com> writes:
> > > > 
> > > > > Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
> > > > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92768
> > > > > ---
> > > > >  drivers/gpu/drm/i915/intel_pm.c | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/i915/intel_pm.c
> > > > > b/drivers/gpu/drm/i915/intel_pm.c
> > > > > index 647c0ff..fc5097f 100644
> > > > > --- a/drivers/gpu/drm/i915/intel_pm.c
> > > > > +++ b/drivers/gpu/drm/i915/intel_pm.c
> > > > > @@ -7138,7 +7138,7 @@ static int chv_freq_opcode(struct
> > > > > drm_i915_private *dev_priv, int val)
> > > > >  
> > > > >  int intel_gpu_freq(struct drm_i915_private *dev_priv, int
> > > > > val)
> > > > >  {
> > > > > -	if (IS_GEN9(dev_priv->dev))
> > > > > +	if (IS_GEN9(dev_priv->dev) && !IS_BROXTON(dev_priv
> > > > > ->dev))
> > > > >  		return (val * GT_FREQUENCY_MULTIPLIER) /
> > > > >  GEN9_FREQ_SCALER;
> > > > 
> > > > Documentation disagrees with this patch. The units are 16.67Mhz
> > > > thus we should use 50/3.
> > > > 
> > > > -Mika
> > > 
> > > I'm not sure I trust the documentation in this case.  Elsewhere,
> > > in
> > > gen6_init_rps_frequencies() we use GEN9_FREQ_SCALER for SKL only,
> > > not for BXT.
> > 
> > On SKL the frequency _capability_ register uses 50MHz units, but
> > the
> > frequency _request_ register uses 16.67MHz units. We store the
> > frequency
> > limits in 16.67MHz units in rps.rp*_freq, hence the use of
> > GEN9_FREQ_SCALER in gen6_init_rps_frequencies(). When requesting a
> > frequency we only use GEN9_FREQ_SCALER to get the 16.67MHz constant
> > value.
> > 
> > On BXT both the capability and request registers use 16.67MHz
> > units, so
> > we don't need to convert in gen6_init_rps_frequencies(), but we
> > still
> > use GEN9_FREQ_SCALER when requesting the frequency to get the
> > 16.67MHz
> > constant value.
> > Confusing like hell, but it works.
> 
> That makes it a bit less confusing, thanks!   Is this all documented
> somewhere?  I'd like to read up on it so if you have any pointers
> please send me an email.

For SKL:
RP_STATE_CAP in configdb (offset 0x5998)
RP_FREQ_NORMAL in bspec (0xA008)

For BXT:
GRAPHICS_FREQUENCY_CAPABILITIES in configdb (offset 0x8170)
RP_FREQ_NORMAL in bspec (0xA008)

--Imre
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