drm: Add TODO item for optimizing format helpers

Add a TODO item for optimizing blitting and format-conversion helpers
in DRM and fbdev. There's always demand for faster graphics output.

v4:
	* fix typos (Sam)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223193804.18636-6-tzimmermann@suse.de

Documentation/gpu/todo.rst

@@ -241,6 +241,28 @@ Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
 
 Level: Advanced
 
+Benchmark and optimize blitting and format-conversion function
+--------------------------------------------------------------
+
+Drawing to dispay memory quickly is crucial for many applications'
+performance.
+
+On at least x86-64, sys_imageblit() is significantly slower than
+cfb_imageblit(), even though both use the same blitting algorithm and
+the latter is written for I/O memory. It turns out that cfb_imageblit()
+uses movl instructions, while sys_imageblit apparently does not. This
+seems to be a problem with gcc's optimizer. DRM's format-conversion
+helpers might be subject to similar issues.
+
+Benchmark and optimize fbdev's sys_() helpers and DRM's format-conversion
+helpers. In cases that can be further optimized, maybe implement a different
+algorithm. For micro-optimizations, use movl/movq instructions explicitly.
+That might possibly require architecture-specific helpers (e.g., storel()
+storeq()).
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>
+
+Level: Intermediate
 
 drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup
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