Re: [PATCH v2 1/4] drm/komeda: Add a new helper drm_color_ctm_s31_32_to_qm_n()

[Mihail Atanassov]

On Tuesday, 15 October 2019 02:16:11 BST james qian wang (Arm Technology China) wrote:
> On Mon, Oct 14, 2019 at 11:58:48AM -0400, Ilia Mirkin wrote:
> > On Fri, Oct 11, 2019 at 1:43 AM james qian wang (Arm Technology China)
> > <james.qian.wang@xxxxxxx> wrote:
> > >
> > > Add a new helper function drm_color_ctm_s31_32_to_qm_n() for driver to
> > > convert S31.32 sign-magnitude to Qm.n 2's complement that supported by
> > > hardware.
> > >
> > > Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@xxxxxxx>
> > > ---
> > >  drivers/gpu/drm/drm_color_mgmt.c | 23 +++++++++++++++++++++++
> > >  include/drm/drm_color_mgmt.h     |  2 ++
> > >  2 files changed, 25 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
> > > index 4ce5c6d8de99..3d533d0b45af 100644
> > > --- a/drivers/gpu/drm/drm_color_mgmt.c
> > > +++ b/drivers/gpu/drm/drm_color_mgmt.c
> > > @@ -132,6 +132,29 @@ uint32_t drm_color_lut_extract(uint32_t user_input, uint32_t bit_precision)
> > >  }
> > >  EXPORT_SYMBOL(drm_color_lut_extract);
> > >
> > > +/**
> > > + * drm_color_ctm_s31_32_to_qm_n
> > > + *
> > > + * @user_input: input value
> > > + * @m: number of integer bits
> > 
> > Is this the full 2's complement value? i.e. including the "sign" bit
> > of the 2's complement representation? I'd kinda assume that m = 32, n
> > = 0 would just get me the integer portion of this, for example.
> 
> @m doesn't include "sign-bit"
> 
> and for this conversion only support m <= 31, n <= 32.
> 
> > > + * @n: number of fractinal bits
> > 
> > fractional
> 
> Thank you.
> > 
> > > + *
> > > + * Convert and clamp S31.32 sign-magnitude to Qm.n 2's complement.
> > > + */
> > > +uint64_t drm_color_ctm_s31_32_to_qm_n(uint64_t user_input,
> > > +                                     uint32_t m, uint32_t n)
> > > +{
> > > +       u64 mag = (user_input & ~BIT_ULL(63)) >> (32 - n);
> > > +       bool negative = !!(user_input & BIT_ULL(63));
> > > +       s64 val;
> > > +
> > > +       /* the range of signed 2s complement is [-2^n+m, 2^n+m - 1] */
> > 
> > This implies that n = 32, m = 0 would actually yield a 33-bit 2's
> > complement number. Is that what you meant?
> 
> Yes, since m doesn't include sign-bit So a Q0.32 is a 33bit value.
> 

I gotta say this would be quite confusing. There is no sign bit in 2's
complement, per se. The MSbit just has a negative weight. Q16.16 is a
32-bit value, so Q0.32 should also be a 32-bit value with weights
-2^-1, +2^-2, etc.

Best to follow what Wikipedia says, right :).

> > 
> > > +       val = clamp_val(mag, 0, negative ? BIT(n + m) : BIT(n + m) - 1);
> > 
> > I'm going to play with numpy to convince myself that this is right
> > (esp with the endpoints), but in the meanwhile, you probably want to
> > use BIT_ULL in case n + m > 32 (I don't think that's the case with any
> > current hardware though).
> 
> Yes, you are right, I need to use BIT_ULL, and Mihail also point this out.
> This is function is drived from our internal s31_32_to_q2_14()
> 
> > 
> > > +
> > > +       return negative ? 0ll - val : val;
> > 
> > Why not just "negative ? -val : val"?
> 
> will correct it.
> 
> > 
> > > +}
> > > +EXPORT_SYMBOL(drm_color_ctm_s31_32_to_qm_n);
> > > +
> > >  /**
> > >   * drm_crtc_enable_color_mgmt - enable color management properties
> > >   * @crtc: DRM CRTC
> > > diff --git a/include/drm/drm_color_mgmt.h b/include/drm/drm_color_mgmt.h
> > > index d1c662d92ab7..60fea5501886 100644
> > > --- a/include/drm/drm_color_mgmt.h
> > > +++ b/include/drm/drm_color_mgmt.h
> > > @@ -30,6 +30,8 @@ struct drm_crtc;
> > >  struct drm_plane;
> > >
> > >  uint32_t drm_color_lut_extract(uint32_t user_input, uint32_t bit_precision);
> > > +uint64_t drm_color_ctm_s31_32_to_qm_n(uint64_t user_input,
> > > +                                     uint32_t m, uint32_t n);
> > >
> > >  void drm_crtc_enable_color_mgmt(struct drm_crtc *crtc,
> > >                                 uint degamma_lut_size,
> > > --
> > > 2.20.1
> > >
> 


-- 
Mihail