Re: [PATCH v5 1/4] drm: Add a new helper drm_color_ctm_s31_32_to_qm_n()

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

On Wednesday, 16 October 2019 11:34:08 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.
> 
> V4: Address Mihai, Daniel and Ilia's review comments.
> V5: Includes the sign bit in the value of m (Qm.n).
> 
> Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
> Reviewed-by: Mihail Atanassov <mihail.atanassov@arm.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/drm_color_mgmt.c | 27 +++++++++++++++++++++++++++
>  include/drm/drm_color_mgmt.h     |  2 ++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
> index 4ce5c6d8de99..d313f194f1ec 100644
> --- a/drivers/gpu/drm/drm_color_mgmt.c
> +++ b/drivers/gpu/drm/drm_color_mgmt.c
> @@ -132,6 +132,33 @@ 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, include the sign-bit, support range is [1, 32]

Any reason why numbers like Q0.32 are disallowed? In those cases, the
'sign' bit and the first fractional bit just happen to be the same bit.
The longer I look at it, the more I think mentioning a 'sign-bit' here
might confuse people more, since 2's complement doesn't have a
dedicated bit just for the sign. How about reducing it simply to:

 * @m: number of integer bits, m <= 32.

> + * @n: number of fractional bits, only support n <= 32
> + *
> + * Convert and clamp S31.32 sign-magnitude to Qm.n (signed 2's complement). The
> + * higher bits that above m + n are cleared or equal to sign-bit BIT(m+n).

[nit] BIT(m + n - 1) if we count from 0.

> + */
> +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;
> +
> +	WARN_ON(m < 1 || m > 32 || n > 32);
> +
> +	/* the range of signed 2's complement is [-2^(m-1), 2^(m-1) - 2^-n] */
> +	val = clamp_val(mag, 0, negative ?
> +				BIT_ULL(n + m - 1) : BIT_ULL(n + m - 1) - 1);
> +
> +	return 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,
> 


-- 
Mihail