Re: [PATCH 2/3] drm/komeda: Add drm_ctm_to_coeffs()

[Daniel Vetter]

On Mon, Sep 30, 2019 at 07:56:13AM -0400, Ilia Mirkin wrote:
> On Mon, Sep 30, 2019 at 7:05 AM Brian Starkey <Brian.Starkey@xxxxxxx> wrote:
> >
> > Hi James,
> >
> > On Mon, Sep 30, 2019 at 04:54:41AM +0000, james qian wang (Arm Technology China) wrote:
> > > This function is used to convert drm_color_ctm S31.32 sign-magnitude
> > > value to komeda required Q2.12 2's complement
> > >
> > > Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@xxxxxxx>
> > > ---
> > >  .../arm/display/komeda/komeda_color_mgmt.c    | 27 +++++++++++++++++++
> > >  .../arm/display/komeda/komeda_color_mgmt.h    |  1 +
> > >  2 files changed, 28 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/arm/display/komeda/komeda_color_mgmt.c b/drivers/gpu/drm/arm/display/komeda/komeda_color_mgmt.c
> > > index c180ce70c26c..c92c82eebddb 100644
> > > --- a/drivers/gpu/drm/arm/display/komeda/komeda_color_mgmt.c
> > > +++ b/drivers/gpu/drm/arm/display/komeda/komeda_color_mgmt.c
> > > @@ -117,3 +117,30 @@ void drm_lut_to_fgamma_coeffs(struct drm_property_blob *lut_blob, u32 *coeffs)
> > >  {
> > >       drm_lut_to_coeffs(lut_blob, coeffs, sector_tbl, ARRAY_SIZE(sector_tbl));
> > >  }
> > > +
> > > +/* Convert from S31.32 sign-magnitude to HW Q2.12 2's complement */
> > > +static s32 drm_ctm_s31_32_to_q2_12(u64 input)
> > > +{
> > > +     u64 mag = (input & ~BIT_ULL(63)) >> 20;
> > > +     bool negative = !!(input & BIT_ULL(63));
> > > +     u32 val;
> > > +
> > > +     /* the range of signed 2s complement is [-2^n, 2^n - 1] */
> > > +     val = clamp_val(mag, 0, negative ? BIT(14) : BIT(14) - 1);
> > > +
> > > +     return negative ? 0 - val : val;
> > > +}
> >
> > This function looks generally useful. Should it be in DRM core
> > (assuming there isn't already one there)?
> >
> > You can use a parameter to determine the number of bits desired in the
> > output.
> 
> I suspect every driver needs to do something similar. You can see what
> I did for nouveau here:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=88b703527ba70659365d989f29579f1292ebf9c3
> 
> (look for csc_drm_to_base)
> 
> Would be great to have a common helper which correctly accounts for
> all the variability.

Yeah the sign-bit 31.32 fixed point format is probably the most ludicrous
uapi fumble we've ever done. A shared function, rolled out to drivers, to
switch it to a signed 2 complements integer sounds like a _very_ good
idea.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch