[PATCH 2/4] drm/format-helper: Add drm_fb_gray8_to_mono_reversed()

[Javier Martinez Canillas <javierm@redhat.com>]

Add support to convert 8-bit grayscale to reversed monochrome for drivers
that control monochromatic displays, that only have 1 bit per pixel depth.

This helper function was based on repaper_gray8_to_mono_reversed() from
the drivers/gpu/drm/tiny/repaper.c driver.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

 drivers/gpu/drm/drm_format_helper.c | 35 +++++++++++++++++++++++++++++
 include/drm/drm_format_helper.h     |  2 ++
 2 files changed, 37 insertions(+)

diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
index 0f28dd2bdd72..bf477c136082 100644
--- a/drivers/gpu/drm/drm_format_helper.c
+++ b/drivers/gpu/drm/drm_format_helper.c
@@ -584,3 +584,38 @@ int drm_fb_blit_toio(void __iomem *dst, unsigned int dst_pitch, uint32_t dst_for
 	return -EINVAL;
 }
 EXPORT_SYMBOL(drm_fb_blit_toio);
+
+/**
+ * drm_fb_gray8_to_mono_reversed - Convert grayscale to reversed monochrome
+ * @dst: reversed monochrome destination buffer
+ * @src: 8-bit grayscale source buffer
+ * @clip: Clip rectangle area to copy
+ *
+ * DRM doesn't have native monochrome or grayscale support.
+ * Such drivers can announce the commonly supported XR24 format to userspace
+ * and use drm_fb_xrgb8888_to_gray8() to convert to grayscale and then this
+ * helper function to convert to the native format.
+ */
+void drm_fb_gray8_to_mono_reversed(void *dst, void *src, const struct drm_rect *clip)
+{
+	size_t width = drm_rect_width(clip);
+	size_t height = drm_rect_width(clip);
+
+	u8 *mono = dst, *gray8 = src;
+	unsigned int y, xb, i;
+
+	for (y = 0; y < height; y++)
+		for (xb = 0; xb < width / 8; xb++) {
+			u8 byte = 0x00;
+
+			for (i = 0; i < 8; i++) {
+				int x = xb * 8 + i;
+
+				byte >>= 1;
+				if (gray8[y * width + x] >> 7)
+					byte |= BIT(7);
+			}
+			*mono++ = byte;
+		}
+}
+EXPORT_SYMBOL(drm_fb_gray8_to_mono_reversed);
diff --git a/include/drm/drm_format_helper.h b/include/drm/drm_format_helper.h
index b30ed5de0a33..cd4c8b7c78de 100644
--- a/include/drm/drm_format_helper.h
+++ b/include/drm/drm_format_helper.h
@@ -43,4 +43,6 @@ int drm_fb_blit_toio(void __iomem *dst, unsigned int dst_pitch, uint32_t dst_for
 		     const void *vmap, const struct drm_framebuffer *fb,
 		     const struct drm_rect *rect);
 
+void drm_fb_gray8_to_mono_reversed(void *dst, void *vaddr, const struct drm_rect *clip);
+
 #endif /* __LINUX_DRM_FORMAT_HELPER_H */
-- 
2.34.1