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From:   Mihail Atanassov <Mihail.Atanassov@arm.com>
To:     "james qian wang (Arm Technology China)" <james.qian.wang@arm.com>
CC:     Liviu Dudau <Liviu.Dudau@arm.com>,
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        "Jonathan Chai (Arm Technology China)" <Jonathan.Chai@arm.com>,
        "Julien Yin (Arm Technology China)" <Julien.Yin@arm.com>,
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        "Lowry Li (Arm Technology China)" <Lowry.Li@arm.com>,
        Ayan Halder <Ayan.Halder@arm.com>,
        "Tiannan Zhu (Arm Technology China)" <Tiannan.Zhu@arm.com>,
        "Yiqi Kang (Arm Technology China)" <Yiqi.Kang@arm.com>,
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        Ben Davis <Ben.Davis@arm.com>,
        "Oscar Zhang (Arm Technology China)" <Oscar.Zhang@arm.com>,
        "Channing Chen (Arm Technology China)" <Channing.Chen@arm.com>
Subject: Re: [PATCH v2 1/4] drm: Add a new helper
 drm_color_ctm_s31_32_to_qm_n()
Thread-Topic: [PATCH v2 1/4] drm: Add a new helper
 drm_color_ctm_s31_32_to_qm_n()
Thread-Index: AQHVf/cdAHid3tH8+UyzZeXX5YE1LKdVLBiAgAS7sICAAA99gA==
Date:   Mon, 14 Oct 2019 10:39:07 +0000
Message-ID: <2848496.aPtRGPrWej@e123338-lin>
References: <20191011054459.17984-1-james.qian.wang@arm.com>
 <1622787.6FTe1jSl1W@e123338-lin> <20191014094332.GA15094@jamwan02-TSP300>
In-Reply-To: <20191014094332.GA15094@jamwan02-TSP300>
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On Monday, 14 October 2019 10:43:39 BST james qian wang (Arm Technology Chi=
na) wrote:
> On Fri, Oct 11, 2019 at 08:26:53AM +0000, Mihail Atanassov wrote:
> > Hi James,
> >=20
> > 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 t=
o
> > > convert S31.32 sign-magnitude to Qm.n 2's complement that supported b=
y
> > > hardware.
> > >=20
> > > Signed-off-by: james qian wang (Arm Technology China) <james.qian.wan=
g@arm.com>
> > > ---
> > >  drivers/gpu/drm/drm_color_mgmt.c | 23 +++++++++++++++++++++++
> > >  include/drm/drm_color_mgmt.h     |  2 ++
> > >  2 files changed, 25 insertions(+)
> > >=20
> > > diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_c=
olor_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_inp=
ut, uint32_t bit_precision)
> > >  }
> > >  EXPORT_SYMBOL(drm_color_lut_extract);
> > >=20
> > > +/**
> > > + * drm_color_ctm_s31_32_to_qm_n
> > > + *
> > > + * @user_input: input value
> > > + * @m: number of integer bits, the m must <=3D 31
> > > + * @n: number of fractinal bits the n must <=3D 32
>=20
> @m: number of integer bits, only support m <=3D 31
> @n: number of fractinal bitsm only support n <=3D 32
>=20

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 =3D (user_input & ~BIT_ULL(63)) >> (32 - n);
> > This doesn't account for n > 32, which is perfectly possible (e.g. Q1.6=
3).
> > > +	bool negative =3D !!(user_input & BIT_ULL(63));
> > > +	s64 val;
> > > +
> > > +	/* the range of signed 2s complement is [-2^n+m, 2^n+m - 1] */
> > > +	val =3D clamp_val(mag, 0, negative ? BIT(n + m) : BIT(n + m) - 1);
> > This also doesn't account for n + m =3D=3D 64.
>=20
> Yes the func is only for support m <=3D 31, n <=3D 32
>=20
> 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.

>=20
> > > +
> > > +	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_mgm=
t.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;
> > >=20
> > >  uint32_t drm_color_lut_extract(uint32_t user_input, uint32_t bit_pre=
cision);
> > > +uint64_t drm_color_ctm_s31_32_to_qm_n(uint64_t user_input,
> > > +				      uint32_t m, uint32_t n);
> > >=20
> > >  void drm_crtc_enable_color_mgmt(struct drm_crtc *crtc,
> > >  				uint degamma_lut_size,
> > > --
> > > 2.20.1
> > >=20
> >=20
> >=20
>=20


--=20
Mihail