doc: media: Document MM21 tiled format

MM21 has been documented through two software implementation [0] in libyuv
and [2] GStreamer. This format is similar to other tiled format, though it
uses uneven tile sizes. The luma tiles are 16x32 while the chroma tile have
a subsampled size of 16x16. This is the uncompressed cousin of
V4L2_PIX_FMT_MT21C and shares its tiling pattern and alignment.

[0] b4ddbaf549
[1] c9b127dae3

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

Documentation/userspace-api/media/v4l/pixfmt-reserved.rst

@@ -233,19 +233,12 @@ please make a proposal on the linux-media mailing list.
 
       - ``V4L2_PIX_FMT_MT21C``
       - 'MT21'
-      - Compressed two-planar YVU420 format used by Mediatek MT8173.
-	The compression is lossless.
-	It is an opaque intermediate format and the MDP hardware must be
+      - Compressed two-planar YVU420 format used by Mediatek MT8173, MT8192,
+        MT8195 and more. The compression is lossless. This format have
+	similitude with ``V4L2_PIX_FMT_MM21`` in term of alignment and tiling.
+	It remains an opaque intermediate format and the MDP hardware must be
 	used to convert ``V4L2_PIX_FMT_MT21C`` to ``V4L2_PIX_FMT_NV12M``,
 	``V4L2_PIX_FMT_YUV420M`` or ``V4L2_PIX_FMT_YVU420``.
-    * .. _V4L2-PIX-FMT-MM21:
-
-      - ``V4L2_PIX_FMT_MM21``
-      - 'MM21'
-      - Non-compressed, tiled two-planar format used by Mediatek MT8183.
-	This is an opaque intermediate format and the MDP3 hardware can be
-	used to convert it to other formats.
-
 .. raw:: latex
 
     \normalsize

Documentation/userspace-api/media/v4l/pixfmt-yuv-planar.rst

@@ -296,6 +296,12 @@ tiles linearly in memory. The line stride and image height must be
 aligned to a multiple of 32. The layouts of the luma and chroma planes are
 identical.
 
+``V4L2_PIX_FMT_MM21`` store luma pixel in 16x32 tiles, and chroma pixels
+in 16x16 tiles. The line stride must be aligned to a multiple of 16 and the
+image height must be aligned to a multiple of 32. The number of luma and chroma
+tiles are identical, even though the tile size differ. The image is formed of
+two non-contiguous planes.
+
 .. _nv12mt:
 
 .. kernel-figure:: nv12mt.svg