Re: [PATCH 4/4] drm/i915: reorganize the get_cdclk assignment

Paulo Zanoni
2017-02-17T15:18:03Z

Em Sex, 2017-02-17 às 16:05 +0200, Ville Syrjälä escreveu:
> On Fri, Feb 17, 2017 at 11:22:07AM -0200, Paulo Zanoni wrote:
> >
> > Possible problems of the current if-ladder:
> >   - It's a huge if ladder with almost a different check for each of
> >     our platforms.
> >   - It mixes 3 different types of checks: IS_GENX, IS_PLATFORM and
> >     IS_GROUP_OF_PLATFORMS.
> >   - As demonstrated by the recent IS_G4X commit, it's not easy to
> > be
> >     sure if a platform down on the list isn't also checked earlier.
> >   - As demonstrated by the WARN at the end, it's not easy to be
> > sure
> >     if we're actually checking for every single platform.
> >
> > Possible advantages of the new switch statement:
> >   - It may be easier for the compiler to optimize stuff (I didn't
> >     check this), especially since the values are labels of an enum.
> >   - The compiler will tell us in case we miss some platform.
> >   - All platforms are explicitly there instead of maybe hidden in
> > some
> >     check for a certain group of platforms such as IS_GEN9_BC.
>
> Performance is a bit of a moot point since this is run exaclty once,
> but
> the IS_GEN9_BC() stuff I tend to agree with. I don't really like
> those
> macros at all since they don't actully mean anything as far as the
> hardware features go.

I think they make some sense when they're a single check. But when we
have tons of checks for tons of platforms, I don't know.

>
> >
> >
> > Possible disadvantages with the new code:
> >   - A few lines bigger.
> >
> > Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_cdclk.c | 103 ++++++++++++++++++++++---
> > ------------
> >  1 file changed, 62 insertions(+), 41 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_cdclk.c
> > b/drivers/gpu/drm/i915/intel_cdclk.c
> > index 7c92dc7..58a2f5c 100644
> > --- a/drivers/gpu/drm/i915/intel_cdclk.c
> > +++ b/drivers/gpu/drm/i915/intel_cdclk.c
> > @@ -1789,49 +1789,70 @@ void intel_init_cdclk_hooks(struct
> > drm_i915_private *dev_priv)
> >   dev_priv->display.modeset_calc_cdclk =
> > skl_modeset_calc_cdclk;
> >   }
> >
> > - if (IS_GEN9_BC(dev_priv))
> > - dev_priv->display.get_cdclk = skl_get_cdclk;
> > - else if (IS_GEN9_LP(dev_priv))
> > - dev_priv->display.get_cdclk = bxt_get_cdclk;
> > - else if (IS_BROADWELL(dev_priv))
> > - dev_priv->display.get_cdclk = bdw_get_cdclk;
> > - else if (IS_HASWELL(dev_priv))
> > - dev_priv->display.get_cdclk = hsw_get_cdclk;
> > - else if (IS_VALLEYVIEW(dev_priv) ||
> > IS_CHERRYVIEW(dev_priv))
> > - dev_priv->display.get_cdclk = vlv_get_cdclk;
> > - else if (IS_GEN6(dev_priv) || IS_IVYBRIDGE(dev_priv))
> > + switch (INTEL_INFO(dev_priv)->platform) {
> > + case INTEL_PLATFORM_UNINITIALIZED:
>
> Just default: ?

If we add a default case the compiler will stop complaining in case we
don't explicitly list every platform. It's a trade-off, I really think
the current way is slightly better, but I won't oppose in case you
still think it's better adding the default case.

>
> >
> > + MISSING_CASE(INTEL_INFO(dev_priv)->platform);
> > + /* Fall through. */
> > + case INTEL_I830:
> > + dev_priv->display.get_cdclk =
> > fixed_133mhz_get_cdclk;
> > + break;
> > + case INTEL_I845G:
> > + dev_priv->display.get_cdclk =
> > fixed_200mhz_get_cdclk;
> > + break;
> > + case INTEL_I85X:
> > + dev_priv->display.get_cdclk = i85x_get_cdclk;
> > + break;
> > + case INTEL_I865G:
> > + dev_priv->display.get_cdclk =
> > fixed_266mhz_get_cdclk;
> > + break;
> > + case INTEL_I915G:
> > + dev_priv->display.get_cdclk =
> > fixed_333mhz_get_cdclk;
> > + break;
> > + case INTEL_I915GM:
> > + dev_priv->display.get_cdclk = i915gm_get_cdclk;
> > + break;
> > + case INTEL_I945G:
> > + case INTEL_I965G:
> > + case INTEL_SANDYBRIDGE:
> > + case INTEL_IVYBRIDGE:
>
> I don't particularly like this disorder. I just managed to get the
> list into some sort of sane order recently.

My original thought here was that since the compiler will actually
complain in case we miss some platform, keeping a strict order is not
as meaningful as it was before. But I was also wondering if this was
actually better or not, so I can change this.

But I did notice you sorted the list. In fact, I originally wrote this
commit against a tree without your improvements, so one of the reasons
I cited in the commit message was the mess of an ordering we had at
that time :).

>
> >
> >   dev_priv->display.get_cdclk =
> > fixed_400mhz_get_cdclk;
> > - else if (IS_GEN5(dev_priv))
> > - dev_priv->display.get_cdclk =
> > fixed_450mhz_get_cdclk;
> > - else if (IS_GM45(dev_priv))
> > - dev_priv->display.get_cdclk = gm45_get_cdclk;
> > - else if (IS_G45(dev_priv))
> > + break;
> > + case INTEL_I945GM:
> > + dev_priv->display.get_cdclk = i945gm_get_cdclk;
> > + break;
> > + case INTEL_G33:
> > + case INTEL_G45:
>
> More disorder.
>
> >
> >   dev_priv->display.get_cdclk = g33_get_cdclk;
> > - else if (IS_I965GM(dev_priv))
> > - dev_priv->display.get_cdclk = i965gm_get_cdclk;
> > - else if (IS_I965G(dev_priv))
> > - dev_priv->display.get_cdclk =
> > fixed_400mhz_get_cdclk;
> > - else if (IS_PINEVIEW(dev_priv))
> > + break;
> > + case INTEL_PINEVIEW:
> >   dev_priv->display.get_cdclk = pnv_get_cdclk;
> > - else if (IS_G33(dev_priv))
> > - dev_priv->display.get_cdclk = g33_get_cdclk;
> > - else if (IS_I945GM(dev_priv))
> > - dev_priv->display.get_cdclk = i945gm_get_cdclk;
> > - else if (IS_I945G(dev_priv))
> > - dev_priv->display.get_cdclk =
> > fixed_400mhz_get_cdclk;
> > - else if (IS_I915GM(dev_priv))
> > - dev_priv->display.get_cdclk = i915gm_get_cdclk;
> > - else if (IS_I915G(dev_priv))
> > - dev_priv->display.get_cdclk =
> > fixed_333mhz_get_cdclk;
> > - else if (IS_I865G(dev_priv))
> > - dev_priv->display.get_cdclk =
> > fixed_266mhz_get_cdclk;
> > - else if (IS_I85X(dev_priv))
> > - dev_priv->display.get_cdclk = i85x_get_cdclk;
> > - else if (IS_I845G(dev_priv))
> > - dev_priv->display.get_cdclk =
> > fixed_200mhz_get_cdclk;
> > - else { /* 830 */
> > - WARN(!IS_I830(dev_priv),
> > -      "Unknown platform. Assuming 133 MHz
> > CDCLK\n");
> > - dev_priv->display.get_cdclk =
> > fixed_133mhz_get_cdclk;
> > + break;
> > + case INTEL_I965GM:
> > + dev_priv->display.get_cdclk = i965gm_get_cdclk;
> > + break;
> > + case INTEL_GM45:
> > + dev_priv->display.get_cdclk = gm45_get_cdclk;
> > + break;
> > + case INTEL_IRONLAKE:
> > + dev_priv->display.get_cdclk =
> > fixed_450mhz_get_cdclk;
> > + break;
> > + case INTEL_VALLEYVIEW:
> > + case INTEL_CHERRYVIEW:
> > + dev_priv->display.get_cdclk = vlv_get_cdclk;
> > + break;
> > + case INTEL_HASWELL:
> > + dev_priv->display.get_cdclk = hsw_get_cdclk;
> > + break;
> > + case INTEL_BROADWELL:
> > + dev_priv->display.get_cdclk = bdw_get_cdclk;
> > + break;
> > + case INTEL_SKYLAKE:
> > + case INTEL_KABYLAKE:
> > + dev_priv->display.get_cdclk = skl_get_cdclk;
> > + break;
> > + case INTEL_BROXTON:
> > + case INTEL_GEMINILAKE:
> > + dev_priv->display.get_cdclk = bxt_get_cdclk;
> > + break;
> >   }
> >  }
> > --
> > 2.7.4
> >
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