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

[Govindapillai, Vinod]

Hi stan

combining two threads..

On Tue, Mar 14, 2023 at 11:50:20AM +0200, Govindapillai, Vinod wrote:
> > Hi Stan
> > 
> > Few other places also do not ceil the result as per the 64631
> > 
> > deinterleave, peakbw, clperchgroup 
> > 
> > Well! I am not sure if this has any real impact, but FYI
> 
> As per BSpec we are now supposed to floor all the calculations, but
> not ceil - bw estimation should be rather pessimistic than optimistic.
> 
> Stan

Yes. But our code use DIV_ROUND_UP in those calculations for . Because you removed the other
round_up calls, i just wanted to bring to your attention. But as I mentioned, it might not have much
significance in reality

On Wed, 2023-03-15 at 09:54 +0200, Lisovskiy, Stanislav wrote:
> 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

I doubt if this is round-up. Because Bspec could always use ceil() as it has used in other places.
But there is no explanation about the magic figure of 500. Again probably considering the
calculation, +500 there might be insignificant there. 

Btw, now I wonder what this patch do is against what is claimed in the commit message. What you are
optimizing is the "supply" not "demand". These changes are in the calculations of available memory
bandwidth. So flooring of the available BW against "round_up" bw demand could cause the SAGV to
choose the higher (lower index) point more.. correct?

BR
Vinod
> 
> > 
> > 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);
> >