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

["james qian wang (Arm Technology China)" <james.qian.wang@arm.com>]

On Fri, Oct 11, 2019 at 08:26:53AM +0000, Mihail Atanassov wrote:
> Hi James,
> 
> On Friday, 11 October 2019 06:45:27 BST james qian wang (Arm Technology China) 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@arm.com>
> > ---
> >  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, the m must <= 31
> > + * @n: number of fractinal bits the n must <= 32

@m: number of integer bits, only support m <= 31
@n: number of fractinal bitsm only support n <= 32

> > + *
> > + * 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);
> This doesn't account for n > 32, which is perfectly possible (e.g. Q1.63).
> > +	bool negative = !!(user_input & BIT_ULL(63));
> > +	s64 val;
> > +
> > +	/* the range of signed 2s complement is [-2^n+m, 2^n+m - 1] */
> > +	val = clamp_val(mag, 0, negative ? BIT(n + m) : BIT(n + m) - 1);
> This also doesn't account for n + m == 64.

Yes the func is only for support m <= 31, n <= 32

But I'm not sure, how to handle the unsupport case ?
Maybe just mention it in Doc is enough.

> > +
> > +	return negative ? 0ll - val : val;
> > +}
> > +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
> 
>