drm/format-helper: Export drm_fb_clip_offset()

Provide a function that computes the offset into a blit destination
buffer. This will allow to move destination-buffer clipping into the
format-helper callers.

v4:
	* add missing '@' for parameter documentation
	* fix typo 'frambuffer'
v2:
	* provide documentation (Sam)
	* return 'unsigned int' (Sam, Noralf)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-2-tzimmermann@suse.de

drivers/gpu/drm/drm_format_helper.c

@@ -17,12 +17,27 @@
 #include <drm/drm_fourcc.h>
 #include <drm/drm_rect.h>
 
-static unsigned int clip_offset(struct drm_rect *clip,
-				unsigned int pitch, unsigned int cpp)
+static unsigned int clip_offset(const struct drm_rect *clip, unsigned int pitch, unsigned int cpp)
 {
 	return clip->y1 * pitch + clip->x1 * cpp;
 }
 
+/**
+ * drm_fb_clip_offset - Returns the clipping rectangles byte-offset in a framebuffer
+ * @pitch: Framebuffer line pitch in byte
+ * @format: Framebuffer format
+ * @clip: Clip rectangle
+ *
+ * Returns:
+ * The byte offset of the clip rectangle's top-left corner within the framebuffer.
+ */
+unsigned int drm_fb_clip_offset(unsigned int pitch, const struct drm_format_info *format,
+				const struct drm_rect *clip)
+{
+	return clip_offset(clip, pitch, format->cpp[0]);
+}
+EXPORT_SYMBOL(drm_fb_clip_offset);
+
 /**
  * drm_fb_memcpy - Copy clip buffer
  * @dst: Destination buffer

include/drm/drm_format_helper.h

@@ -6,9 +6,13 @@
 #ifndef __LINUX_DRM_FORMAT_HELPER_H
 #define __LINUX_DRM_FORMAT_HELPER_H
 
+struct drm_format_info;
 struct drm_framebuffer;
 struct drm_rect;
 
+unsigned int drm_fb_clip_offset(unsigned int pitch, const struct drm_format_info *format,
+				const struct drm_rect *clip);
+
 void drm_fb_memcpy(void *dst, void *vaddr, struct drm_framebuffer *fb,
 		   struct drm_rect *clip);
 void drm_fb_memcpy_dstclip(void __iomem *dst, unsigned int dst_pitch, void *vaddr,