[PATCH v4 2/6] drm/format-helper: Add drm_fb_xrgb8888_to_mono_reversed()

[Javier Martinez Canillas]

Add support to convert from XR24 to reversed monochrome for drivers that
control monochromatic display panels, that only have 1 bit per pixel.

The function does a line-by-line conversion doing an intermediate step
first from XR24 to 8-bit grayscale and then to reversed monochrome.

The drm_fb_gray8_to_mono_reversed_line() helper was based on code from
drivers/gpu/drm/tiny/repaper.c driver.

Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
---

Changes in v4:
- Rename end_offset to end_len (Thomas Zimmermann)
- Warn once if dst_pitch is not a multiple of 8 (Thomas Zimmermann)
- Drop drm_fb_gray8_to_mono_reversed() that's not used (Thomas Zimmermann)
- Allocate single buffer for both copy cma memory and gray8 (Thomas Zimmermann)
- Add Thomas Zimmermann Reviewed-by tag to patch adding XR24 -> mono helper.

Changes in v3:
- Also add a drm_fb_xrgb8888_to_mono_reversed() helper (Thomas Zimmermann)
- Split lines copy to drm_fb_gray8_to_mono_reversed_line() (Thomas Zimmermann)
- Handle case where the source buffer is not aligned to 8 (Thomas Zimmermann)

 drivers/gpu/drm/drm_format_helper.c | 107 ++++++++++++++++++++++++++++
 include/drm/drm_format_helper.h     |   4 ++
 2 files changed, 111 insertions(+)

diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
index b981712623d3..ec4e3724ee79 100644
--- a/drivers/gpu/drm/drm_format_helper.c
+++ b/drivers/gpu/drm/drm_format_helper.c
@@ -591,3 +591,110 @@ int drm_fb_blit_toio(void __iomem *dst, unsigned int dst_pitch, uint32_t dst_for
 	return -EINVAL;
 }
 EXPORT_SYMBOL(drm_fb_blit_toio);
+
+static void drm_fb_gray8_to_mono_reversed_line(u8 *dst, const u8 *src, unsigned int pixels,
+					       unsigned int start_offset, unsigned int end_len)
+{
+	unsigned int xb, i;
+
+	for (xb = 0; xb < pixels; xb++) {
+		unsigned int start = 0, end = 8;
+		u8 byte = 0x00;
+
+		if (xb == 0 && start_offset)
+			start = start_offset;
+
+		if (xb == pixels - 1 && end_len)
+			end = end_len;
+
+		for (i = start; i < end; i++) {
+			unsigned int x = xb * 8 + i;
+
+			byte >>= 1;
+			if (src[x] >> 7)
+				byte |= BIT(7);
+		}
+		*dst++ = byte;
+	}
+}
+
+/**
+ * drm_fb_xrgb8888_to_mono_reversed - Convert XRGB8888 to reversed monochrome
+ * @dst: reversed monochrome destination buffer
+ * @dst_pitch: Number of bytes between two consecutive scanlines within dst
+ * @src: XRGB8888 source buffer
+ * @fb: DRM framebuffer
+ * @clip: Clip rectangle area to copy
+ *
+ * DRM doesn't have native monochrome support.
+ * Such drivers can announce the commonly supported XR24 format to userspace
+ * and use this function to convert to the native format.
+ *
+ * This function uses drm_fb_xrgb8888_to_gray8() to convert to grayscale and
+ * then the result is converted from grayscale to reversed monohrome.
+ */
+void drm_fb_xrgb8888_to_mono_reversed(void *dst, unsigned int dst_pitch, const void *vaddr,
+				      const struct drm_framebuffer *fb, const struct drm_rect *clip)
+{
+	unsigned int linepixels = drm_rect_width(clip);
+	unsigned int lines = clip->y2 - clip->y1;
+	unsigned int cpp = fb->format->cpp[0];
+	unsigned int len_src32 = linepixels * cpp;
+	unsigned int start_offset, end_len;
+	unsigned int y;
+	u8 *mono = dst, *gray8;
+	u32 *src32;
+
+	if (WARN_ON(fb->format->format != DRM_FORMAT_XRGB8888))
+		return;
+
+	/*
+	 * The reversed mono destination buffer contains 1 bit per pixel
+	 * and destination scanlines have to be in multiple of 8 pixels.
+	 */
+	if (!dst_pitch)
+		dst_pitch = DIV_ROUND_UP(linepixels, 8);
+
+	WARN_ONCE(dst_pitch % 8 != 0, "dst_pitch is not a multiple of 8\n");
+
+	/*
+	 * The cma memory is write-combined so reads are uncached.
+	 * Speed up by fetching one line at a time.
+	 *
+	 * Also, format conversion from XR24 to reversed monochrome
+	 * are done line-by-line but are converted to 8-bit grayscale
+	 * as an intermediate step.
+	 *
+	 * Allocate a buffer to be used for both copying from the cma
+	 * memory and to store the intermediate grayscale line pixels.
+	 */
+	src32 = kmalloc(len_src32 + linepixels, GFP_KERNEL);
+	if (!src32)
+		return;
+
+	gray8 = (u8 *)src32 + len_src32;
+
+	/*
+	 * For damage handling, it is possible that only parts of the source
+	 * buffer is copied and this could lead to start and end pixels that
+	 * are not aligned to multiple of 8.
+	 *
+	 * Calculate if the start and end pixels are not aligned and set the
+	 * offsets for the reversed mono line conversion function to adjust.
+	 */
+	start_offset = clip->x1 % 8;
+	end_len = clip->x2 % 8;
+
+	vaddr += clip_offset(clip, fb->pitches[0], cpp);
+	for (y = 0; y < lines; y++) {
+		src32 = memcpy(src32, vaddr, len_src32);
+		drm_fb_xrgb8888_to_gray8_line(gray8, src32, linepixels);
+		drm_fb_gray8_to_mono_reversed_line(mono, gray8, dst_pitch,
+						   start_offset, end_len);
+		vaddr += fb->pitches[0];
+		mono += dst_pitch;
+	}
+
+	kfree(src32);
+}
+EXPORT_SYMBOL(drm_fb_xrgb8888_to_mono_reversed);
diff --git a/include/drm/drm_format_helper.h b/include/drm/drm_format_helper.h
index b30ed5de0a33..0b0937c0b2f6 100644
--- a/include/drm/drm_format_helper.h
+++ b/include/drm/drm_format_helper.h
@@ -43,4 +43,8 @@ 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_xrgb8888_to_mono_reversed(void *dst, unsigned int dst_pitch, const void *src,
+				      const struct drm_framebuffer *fb,
+				      const struct drm_rect *clip);
+
 #endif /* __LINUX_DRM_FORMAT_HELPER_H */
-- 
2.34.1