[PATCH v2 8/8] drm: vkms: Add support to the RGB565 format

[Igor Torrente <igormtorrente@gmail.com>]

Adds this common format to vkms.

This commit also adds new helper macros to deal with fixed-point
arithmetic.

It was done to improve the precision of the conversion to ARGB16161616
since the "conversion ratio" is not an integer.

Signed-off-by: Igor Torrente <igormtorrente@gmail.com>
---
 drivers/gpu/drm/vkms/vkms_composer.c  |  4 ++
 drivers/gpu/drm/vkms/vkms_formats.h   | 72 +++++++++++++++++++++++++++
 drivers/gpu/drm/vkms/vkms_plane.c     |  6 ++-
 drivers/gpu/drm/vkms/vkms_writeback.c |  3 +-
 4 files changed, 82 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index f16fcfc88cea..57ec82839a89 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -166,6 +166,8 @@ static void ((*get_line_fmt_transform_function(u32 format))
 		return &get_ARGB16161616;
 	else if (format == DRM_FORMAT_XRGB16161616)
 		return &XRGB16161616_to_ARGB16161616;
+	else if (format == DRM_FORMAT_RGB565)
+		return &RGB565_to_ARGB16161616;
 	else
 		return &XRGB8888_to_ARGB16161616;
 }
@@ -179,6 +181,8 @@ static void ((*get_output_line_function(u32 format))
 		return &convert_to_ARGB16161616;
 	else if (format == DRM_FORMAT_XRGB16161616)
 		return &convert_to_XRGB16161616;
+	else if (format == DRM_FORMAT_RGB565)
+		return &convert_to_RGB565;
 	else
 		return &convert_to_XRGB8888;
 }
diff --git a/drivers/gpu/drm/vkms/vkms_formats.h b/drivers/gpu/drm/vkms/vkms_formats.h
index aa433edd00bd..1e2db1a844aa 100644
--- a/drivers/gpu/drm/vkms/vkms_formats.h
+++ b/drivers/gpu/drm/vkms/vkms_formats.h
@@ -8,6 +8,26 @@
 #define pixel_offset(composer, x, y) \
 	((composer)->offset + ((y) * (composer)->pitch) + ((x) * (composer)->cpp))
 
+/*
+ * FP stands for _Fixed Point_ and **not** _Float Point_
+ * LF stands for Long Float (i.e. double)
+ * The following macros help doing fixed point arithmetic.
+ */
+/*
+ * With FP scale 15 we have 17 and 15 bits of integer and fractional parts
+ * respectively.
+ *  | 0000 0000 0000 0000 0.000 0000 0000 0000 |
+ * 31                                          0
+ */
+#define FP_SCALE 15
+
+#define LF_TO_FP(a) ((a) * (u64)(1 << FP_SCALE))
+#define INT_TO_FP(a) ((a) << FP_SCALE)
+#define FP_MUL(a, b) ((s32)(((s64)(a) * (b)) >> FP_SCALE))
+#define FP_DIV(a, b) ((s32)(((s64)(a) << FP_SCALE) / (b)))
+/* This macro converts a fixed point number to int, and round half up it */
+#define FP_TO_INT_ROUND_UP(a) (((a) + (1 << (FP_SCALE - 1))) >> FP_SCALE)
+
 /*
  * packed_pixels_addr - Get the pointer to pixel of a given pair of coordinates
  *
@@ -102,6 +122,35 @@ static void XRGB16161616_to_ARGB16161616(void *pixels_addr, int length,
 	}
 }
 
+static void RGB565_to_ARGB16161616(void *pixels_addr, int length,
+				   u64 *line_buffer)
+{
+	__le16 *src_pixels = pixels_addr;
+	int i;
+
+	for (i = 0; i < length; i++) {
+		u16 rgb_565 = le16_to_cpu(*src_pixels);
+		int fp_r = INT_TO_FP((rgb_565 >> 11) & 0x1f);
+		int fp_g = INT_TO_FP((rgb_565 >> 5) & 0x3f);
+		int fp_b = INT_TO_FP(rgb_565 & 0x1f);
+
+		/*
+		 * The magic constants is the "conversion ratio" and is calculated
+		 * dividing 65535(2^16 - 1) by 31(2^5 -1) and 63(2^6 - 1) respectively.
+		 */
+		int fp_rb_ratio = LF_TO_FP(2114.032258065);
+		int fp_g_ratio = LF_TO_FP(1040.238095238);
+
+		u64 r = FP_TO_INT_ROUND_UP(FP_MUL(fp_r, fp_rb_ratio));
+		u64 g = FP_TO_INT_ROUND_UP(FP_MUL(fp_g, fp_g_ratio));
+		u64 b = FP_TO_INT_ROUND_UP(FP_MUL(fp_b, fp_rb_ratio));
+
+		line_buffer[i] = 0xffffllu << 48 | r << 32 | g << 16 | b;
+
+		src_pixels++;
+	}
+}
+
 /*
  * The following functions are used as blend operations. But unlike the
  * `alpha_blend`, these functions take an ARGB16161616 pixel from the
@@ -177,4 +226,27 @@ static void convert_to_XRGB16161616(void *pixels_addr, int length,
 	}
 }
 
+static void convert_to_RGB565(void *pixels_addr, int length,
+			      u64 *line_buffer)
+{
+	__le16 *dst_pixels = pixels_addr;
+	int i;
+
+	for (i = 0; i < length; i++)  {
+		int fp_r = INT_TO_FP((line_buffer[i] >> 32) & 0xffff);
+		int fp_g = INT_TO_FP((line_buffer[i] >> 16) & 0xffff);
+		int fp_b = INT_TO_FP(line_buffer[i] & 0xffffllu);
+
+		int fp_rb_ratio = LF_TO_FP(2114.032258065);
+		int fp_g_ratio = LF_TO_FP(1040.238095238);
+
+		u16 r = FP_TO_INT_ROUND_UP(FP_DIV(fp_r, fp_rb_ratio));
+		u16 g = FP_TO_INT_ROUND_UP(FP_DIV(fp_g, fp_g_ratio));
+		u16 b = FP_TO_INT_ROUND_UP(FP_DIV(fp_b, fp_rb_ratio));
+
+		*dst_pixels = cpu_to_le16(r << 11 | g << 5 | b);
+		dst_pixels++;
+	}
+}
+
 #endif /* _VKMS_FORMATS_H_ */
diff --git a/drivers/gpu/drm/vkms/vkms_plane.c b/drivers/gpu/drm/vkms/vkms_plane.c
index 516e48b38806..de250808aa39 100644
--- a/drivers/gpu/drm/vkms/vkms_plane.c
+++ b/drivers/gpu/drm/vkms/vkms_plane.c
@@ -13,14 +13,16 @@
 
 static const u32 vkms_formats[] = {
 	DRM_FORMAT_XRGB8888,
-	DRM_FORMAT_XRGB16161616
+	DRM_FORMAT_XRGB16161616,
+	DRM_FORMAT_RGB565
 };
 
 static const u32 vkms_plane_formats[] = {
 	DRM_FORMAT_ARGB8888,
 	DRM_FORMAT_XRGB8888,
 	DRM_FORMAT_XRGB16161616,
-	DRM_FORMAT_ARGB16161616
+	DRM_FORMAT_ARGB16161616,
+	DRM_FORMAT_RGB565
 };
 
 static struct drm_plane_state *
diff --git a/drivers/gpu/drm/vkms/vkms_writeback.c b/drivers/gpu/drm/vkms/vkms_writeback.c
index 0f7bb77f981e..11eb1be5a0fc 100644
--- a/drivers/gpu/drm/vkms/vkms_writeback.c
+++ b/drivers/gpu/drm/vkms/vkms_writeback.c
@@ -15,7 +15,8 @@
 static const u32 vkms_wb_formats[] = {
 	DRM_FORMAT_XRGB8888,
 	DRM_FORMAT_XRGB16161616,
-	DRM_FORMAT_ARGB16161616
+	DRM_FORMAT_ARGB16161616,
+	DRM_FORMAT_RGB565
 };
 
 static const struct drm_connector_funcs vkms_wb_connector_funcs = {
-- 
2.30.2