[Intel-gfx] [PATCH] drm/i915: Power management SAGV/QGV calculation rounding fix

[Lisovskiy, Stanislav]

On Tue, Mar 14, 2023 at 11:27:35AM +0200, Govindapillai, Vinod wrote:
> Hi Stan,
> 
> 
> 
> On Fri, 2023-03-10 at 16:40 +0200, Stanislav Lisovskiy wrote:
> > Currently in our bandwidth calculations for QGV
> > points we round up the calculations.
> > Recently there was an update to BSpec, recommending
> > to floor those calculations.
> > The reasoning behind this was that, overestimating
> > BW demand with that rounding can cause SAGV to use
> > next QGV point, even though the less demanding could
> > be used, thus resulting in waste of power.
> > 
> > BSpec: 64636
> > 
> > Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_bw.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_bw.c b/drivers/gpu/drm/i915/display/intel_bw.c
> > index 202321ffbe2a..8723ddd4d568 100644
> > --- a/drivers/gpu/drm/i915/display/intel_bw.c
> > +++ b/drivers/gpu/drm/i915/display/intel_bw.c
> > @@ -50,7 +50,7 @@ static int dg1_mchbar_read_qgv_point_info(struct drm_i915_private *dev_priv,
> >                 dclk_reference = 6; /* 6 * 16.666 MHz = 100 MHz */
> >         else
> >                 dclk_reference = 8; /* 8 * 16.666 MHz = 133 MHz */
> > -       sp->dclk = DIV_ROUND_UP((16667 * dclk_ratio * dclk_reference) + 500, 1000);
> > +       sp->dclk = ((16667 * dclk_ratio * dclk_reference) + 500) / 1000;
> >  
> >         val = intel_uncore_read(&dev_priv->uncore, SKL_MC_BIOS_DATA_0_0_0_MCHBAR_PCU);
> >         if (val & DG1_GEAR_TYPE)
> > @@ -87,7 +87,7 @@ static int icl_pcode_read_qgv_point_info(struct drm_i915_private *dev_priv,
> >                 return ret;
> >  
> >         dclk = val & 0xffff;
> > -       sp->dclk = DIV_ROUND_UP((16667 * dclk) + (DISPLAY_VER(dev_priv) > 11 ? 500 : 0), 1000);
> > +       sp->dclk = ((16667 * dclk) + (DISPLAY_VER(dev_priv) > 11 ? 500 : 0)) / 1000;
> >         sp->t_rp = (val & 0xff0000) >> 16;
> >         sp->t_rcd = (val & 0xff000000) >> 24;
> >  
> > @@ -179,7 +179,7 @@ static int mtl_read_qgv_point_info(struct drm_i915_private *dev_priv,
> >         val2 = intel_uncore_read(&dev_priv->uncore,
> >                                  MTL_MEM_SS_INFO_QGV_POINT_HIGH(point));
> >         dclk = REG_FIELD_GET(MTL_DCLK_MASK, val);
> > -       sp->dclk = DIV_ROUND_UP((16667 * dclk), 1000);
> > +       sp->dclk = (16667 * dclk) / 1000;
> 
> Not related to this patch. But as per Bspec 64631 and 64636
>   sp->dclk = (16667 * dclk + 500) / 1000;
> 
> Does that need to be corrected as well?

It looks like again rounding up stuff, we divide by 1000,
so we obviously add 500 just to round up by one the end result.
But considering the recent instructions, that we must "floor/int"
all the results, I would say this needs to be corrected in BSpec,
rather than here(wondering why it was implemented initially that way
here, could be that BSpec was initially like this or the one doing
that here was actually smarter than BSpec :) )

Stan

> 
> BR
> vinod
> 
> 
> >         sp->t_rp = REG_FIELD_GET(MTL_TRP_MASK, val);
> >         sp->t_rcd = REG_FIELD_GET(MTL_TRCD_MASK, val);
> >  
> > @@ -425,7 +425,7 @@ static int icl_get_bw_info(struct drm_i915_private *dev_priv, const struct
> > intel
> >                          */
> >                         ct = max_t(int, sp->t_rc, sp->t_rp + sp->t_rcd +
> >                                    (clpchgroup - 1) * qi.t_bl + sp->t_rdpre);
> > -                       bw = DIV_ROUND_UP(sp->dclk * clpchgroup * 32 * num_channels, ct);
> > +                       bw = (sp->dclk * clpchgroup * 32 * num_channels) / ct;
> >  
> >                         bi->deratedbw[j] = min(maxdebw,
> >                                                bw * (100 - sa->derating) / 100);
> > @@ -527,7 +527,7 @@ static int tgl_get_bw_info(struct drm_i915_private *dev_priv, const struct
> > intel
> >                          */
> >                         ct = max_t(int, sp->t_rc, sp->t_rp + sp->t_rcd +
> >                                    (clpchgroup - 1) * qi.t_bl + sp->t_rdpre);
> > -                       bw = DIV_ROUND_UP(sp->dclk * clpchgroup * 32 * num_channels, ct);
> > +                       bw = (sp->dclk * clpchgroup * 32 * num_channels) / ct;
> >  
> >                         bi->deratedbw[j] = min(maxdebw,
> >                                                bw * (100 - sa->derating) / 100);
>