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From:   Ilia Mirkin <imirkin@alum.mit.edu>
Date:   Mon, 14 Oct 2019 11:58:48 -0400
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Subject: Re: [PATCH v2 1/4] drm/komeda: Add a new helper drm_color_ctm_s31_32_to_qm_n()
To:     "james qian wang (Arm Technology China)" <james.qian.wang@arm.com>
Cc:     Liviu Dudau <Liviu.Dudau@arm.com>,
        "airlied@linux.ie" <airlied@linux.ie>,
        Brian Starkey <Brian.Starkey@arm.com>,
        "maarten.lankhorst@linux.intel.com" 
        <maarten.lankhorst@linux.intel.com>,
        "sean@poorly.run" <sean@poorly.run>,
        "Jonathan Chai (Arm Technology China)" <Jonathan.Chai@arm.com>,
        "Julien Yin (Arm Technology China)" <Julien.Yin@arm.com>,
        "Thomas Sun (Arm Technology China)" <thomas.Sun@arm.com>,
        "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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        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        Ben Davis <Ben.Davis@arm.com>,
        "Oscar Zhang (Arm Technology China)" <Oscar.Zhang@arm.com>,
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        Mihail Atanassov <Mihail.Atanassov@arm.com>
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On Fri, Oct 11, 2019 at 1:43 AM james qian wang (Arm Technology China)
<james.qian.wang@arm.com> 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

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.

> + * @n: number of fractinal bits

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);
> +       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?

> +       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).

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

Why not just "negative ? -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
>