Rework comments by splitting them into README and removing redundant ones

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>

README.md

@@ -25,3 +25,46 @@ Sample media files can be obtained from:
 
 	http://samplemedia.linaro.org/MPEG2/
 	http://samplemedia.linaro.org/MPEG4/SVT/
+
+## Technical Notes
+
+### Surface
+
+A Surface is an internal data structure never handled by the VA's user
+containing the output of a rendering. Usualy, a bunch of surfaces are created
+at the begining of decoding and they are then used alternatively. When
+created, a surface is assigned a corresponding v4l capture buffer and it is
+kept until the end of decoding. Syncing a surface waits for the v4l buffer to
+be available and then dequeue it.
+
+Note: since a Surface is kept private from the VA's user, it can ask to
+directly render a Surface on screen in an X Drawable. Some kind of
+implementation is available in PutSurface but this is only for development
+purpose.
+
+### Context
+
+A Context is a global data structure used for rendering a video of a certain
+format. When a context is created, input buffers are created and v4l's output
+(which is the compressed data input queue, since capture is the real output)
+format is set.
+
+### Picture
+
+A Picture is an encoded input frame made of several buffers. A single input
+can contain slice data, headers and IQ matrix. Each Picture is assigned a
+request ID when created and each corresponding buffer might be turned into a
+v4l buffers or extended control when rendered. Finally they are submitted to
+kernel space when reaching EndPicture.
+
+The real rendering is done in EndPicture instead of RenderPicture
+because the v4l2 driver expects to have the full corresponding
+extended control when a buffer is queued and we don't know in which
+order the different RenderPicture will be called.
+
+### Image
+
+An Image is a standard data structure containing rendered frames in a usable
+pixel format. Here we only use NV12 buffers which are converted from sunxi's
+proprietary tiled pixel format with tiled_yuv when deriving an Image from a
+Surface.