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

[james qian wang (Arm Technology China)]

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@xxxxxxx>
Reviewed-by: Mihail Atanassov <mihail.atanassov@xxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
---
 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]
+ * @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).
+ */
+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,
-- 
2.20.1