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

[Mihail Atanassov <Mihail.Atanassov@arm.com>]

On Monday, 14 October 2019 10:43:39 BST james qian wang (Arm Technology China) wrote:
> 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
> 

Hehe, guess what I didn't do? Also, [nit], s/fractinal/fractional/.

> > > + *
> > > + * 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.

I'd personally appreciate a WARN_ON(...)*. My comment was motivated by
the unchecked nature of the limitation, which might surprise
a dev who didn't read the doc before using the function (like myself :)).

Actually, the limitations are more than specified, BIT(n + m) is only
valid for (n + m < 32) on LP32 systems.

At least change those to BIT_ULL(), then:

Reviewed-by: Mihail Atanassov <mihail.atanassov@arm.com>

* - gcc9.2 doesn't give me any compile-time warnings for using this func
incorrectly.

> 
> > > +
> > > +	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