[PATCH] drm/amd/display: Interpret log_2(0) as 0

[Marek Olšák]

num_pkrs is a hardware config parameter like the number of shader engines.
It's constant for each chip.

Marek

On Wed, Oct 21, 2020 at 12:39 PM Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
wrote:

> On Wed, Oct 21, 2020 at 6:09 PM Harry Wentland <harry.wentland at amd.com>
> wrote:
> >
> > Even though log(0) is technically undefined our code assumes that
> > log_2(0) is 0. This mirrors the current behavior of our log_2
> > implementation on non-Linux platforms.
> >
> > Signed-off-by: Harry Wentland <harry.wentland at amd.com>
> > ---
> >
> > What's num_pkrs value is upstream Mesa providing for CHIP_VANGOGH?
>
> So mesa isn't quite providing a num_pkrs value to the HW as the render
> HW doesn't require you to set one, it assumes something.
>
> In practice mesa still uses num_pkrs, which is provided by a readback
> of the GB_ADDR_CFG(CONFIG?) register info provided by the kernel. This
> can be used for e.g. determining texture coordinates (like we do for
> the displayable DCC transform) but otherwise isn't really needed,
> since the tile size for texture allocation is not dependent on
> num_pkrs.
>
> so outside of displayable DCC I think you'd be able to get far in mesa
> with a wrong num_pkrs.
>
> >
> > I saw that problem at bringup with an internal Mesa and had this fix
> > for it.
> >
> > Harry
> >
> >  drivers/gpu/drm/amd/display/dc/basics/conversion.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/amd/display/dc/basics/conversion.h
> b/drivers/gpu/drm/amd/display/dc/basics/conversion.h
> > index ade785c4fdc7..da9883ec7b1c 100644
> > --- a/drivers/gpu/drm/amd/display/dc/basics/conversion.h
> > +++ b/drivers/gpu/drm/amd/display/dc/basics/conversion.h
> > @@ -40,7 +40,11 @@ void convert_float_matrix(
> >
> >  static inline unsigned int log_2(unsigned int num)
> >  {
> > -       return ilog2(num);
> > +       /*
> > +        * Technically log(0) is undefined, but our code is structured
> > +        * in a way that assumes log(0) = 0
> > +        */
> > +       return num ? ilog2(num) : num;
> >  }
> >
> >  #endif
> > --
> > 2.28.0
> >
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