[RFC PATCH v2 3/3] drm/afbc: Add AFBC modifier usage documentation

Brian Starkey
2018-08-23T15:24:05Z

AFBC is a flexible, proprietary, lossless compression protocol and
format, with a number of defined DRM format modifiers. To facilitate
consistency and compatibility between different AFBC producers and
consumers, document the expectations for usage of the AFBC DRM format
modifiers in a new .rst chapter.

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
---
Documentation/gpu/afbc.rst | 226 ++++++++++++++++++++++++++++++++++++++++++
Documentation/gpu/drivers.rst | 1 +
MAINTAINERS | 1 +
include/uapi/drm/drm_fourcc.h | 3 +
4 files changed, 231 insertions(+)
create mode 100644 Documentation/gpu/afbc.rst

diff --git a/Documentation/gpu/afbc.rst b/Documentation/gpu/afbc.rst
new file mode 100644
index 000000000000..58d2b6b5d48b
--- /dev/null
+++ b/Documentation/gpu/afbc.rst
@@ -0,0 +1,226 @@
+===================================
+ Arm Framebuffer Compression (AFBC)
+===================================
+
+AFBC is a proprietary lossless image compression protocol and format.
+It provides fine-grained random access and minimizes the amount of
+data transferred between IP blocks.
+
+AFBC can be enabled on drivers which support it via use of the AFBC
+format modifiers defined in drm_fourcc.h. See DRM_FORMAT_MOD_ARM_AFBC(*).
+
+All users of the AFBC modifiers must follow the usage guidelines laid
+out in this document, to ensure compatibility across different AFBC
+producers and consumers.
+
+Components and Ordering
+=======================
+
+AFBC streams can contain several components - where a component
+corresponds to a color channel (i.e. R, G, B, X, A, Y, Cb, Cr).
+The assignment of input/output color channels must be consistent
+between the encoder and the decoder for correct operation, otherwise
+the consumer will interpret the decoded data incorrectly.
+
+Furthermore, when the lossless colorspace transform is used
+(AFBC_FORMAT_MOD_YTR, which should be enabled for RGB buffers for
+maximum compression efficiency), the component order must be:
+
+ * Component 0: R
+ * Component 1: G
+ * Component 2: B
+
+The component ordering is communicated via the fourcc code in the
+fourcc:modifier pair. In general, component '0' is considered to
+reside in the least-significant bits of the corresponding linear
+format. For example, COMP(bits):
+
+ * DRM_FORMAT_ABGR8888
+
+ * Component 0: R(8)
+ * Component 1: G(8)
+ * Component 2: B(8)
+ * Component 3: A(8)
+
+ * DRM_FORMAT_BGR888
+
+ * Component 0: R(8)
+ * Component 1: G(8)
+ * Component 2: B(8)
+
+ * DRM_FORMAT_YUYV
+
+ * Component 0: Y(8)
+ * Component 1: Cb(8, 2x1 subsampled)
+ * Component 2: Cr(8, 2x1 subsampled)
+
+In AFBC, 'X' components are not treated any differently from any other
+component. Therefore, an AFBC buffer with fourcc DRM_FORMAT_XBGR8888
+encodes with 4 components, like so:
+
+ * DRM_FORMAT_XBGR8888
+
+ * Component 0: R(8)
+ * Component 1: G(8)
+ * Component 2: B(8)
+ * Component 3: X(8)
+
+Please note, however, that the inclusion of a "wasted" 'X' channel is
+bad for compression efficiency, and so it's recommended to avoid
+formats containing 'X' bits. If a fourth component is
+required/expected by the encoder/decoder, then it is recommended to
+instead use an equivalent format with alpha, setting all alpha bits to
+'1'. If there is no requirement for a fourth component, then a format
+which doesn't include alpha can be used, e.g. DRM_FORMAT_BGR888.
+
+Number of Planes
+================
+
+Formats which are typically multi-planar in linear layouts (e.g. YUV
+420), can be encoded into one, or multiple, AFBC planes. As with
+component order, the encoder and decoder must agree about the number
+of planes in order to correctly decode the buffer. The fourcc code is
+used to determine the number of encoded planes in an AFBC buffer,
+matching the number of planes for the linear (unmodified) format.
+Within each plane, the component ordering also follows the fourcc
+code:
+
+For example:
+
+ * DRM_FORMAT_YUYV: nplanes = 1
+
+ * Plane 0:
+
+ * Component 0: Y(8)
+ * Component 1: Cb(8, 2x1 subsampled)
+ * Component 2: Cr(8, 2x1 subsampled)
+
+ * DRM_FORMAT_NV12: nplanes = 2
+
+ * Plane 0:
+
+ * Component 0: Y(8)
+
+ * Plane 1:
+
+ * Component 0: Cb(8, 2x1 subsampled)
+ * Component 1: Cr(8, 2x1 subsampled)
+
+Cross-device interoperability
+=============================
+
+For maximum compatibility across devices, the table below defines
+canonical formats for use between AFBC-enabled devices. Formats which
+are listed here must be used exactly as specified when using the AFBC
+modifiers. Formats which are not listed should be avoided.
+
+.. flat-table:: AFBC formats
+
+ * - Fourcc code
+ - Description
+ - Planes/Components
+
+ * - DRM_FORMAT_ABGR2101010
+ - 10-bit per component RGB, with 2-bit alpha
+ - Plane 0: 4 components
+ * Component 0: R(10)
+ * Component 1: G(10)
+ * Component 2: B(10)
+ * Component 3: A(2)
+
+ * - DRM_FORMAT_ABGR8888
+ - 8-bit per component RGB, with 8-bit alpha
+ - Plane 0: 4 components
+ * Component 0: R(8)
+ * Component 1: G(8)
+ * Component 2: B(8)
+ * Component 3: A(8)
+
+ * - DRM_FORMAT_BGR888
+ - 8-bit per component RGB
+ - Plane 0: 3 components
+ * Component 0: R(8)
+ * Component 1: G(8)
+ * Component 2: B(8)
+
+ * - DRM_FORMAT_BGR565
+ - 5/6-bit per component RGB
+ - Plane 0: 3 components
+ * Component 0: R(5)
+ * Component 1: G(6)
+ * Component 2: B(5)
+
+ * - DRM_FORMAT_ABGR1555
+ - 5-bit per component RGB, with 1-bit alpha
+ - Plane 0: 4 components
+ * Component 0: R(5)
+ * Component 1: G(5)
+ * Component 2: B(5)
+ * Component 3: A(1)
+
+ * - DRM_FORMAT_VUY888
+ - 8-bit per component YCbCr 444, single plane
+ - Plane 0: 3 components
+ * Component 0: Y(8)
+ * Component 1: Cb(8)
+ * Component 2: Cr(8)
+
+ * - DRM_FORMAT_VUY101010
+ - 10-bit per component YCbCr 444, single plane
+ - Plane 0: 3 components
+ * Component 0: Y(10)
+ * Component 1: Cb(10)
+ * Component 2: Cr(10)
+
+ * - DRM_FORMAT_YUYV
+ - 8-bit per component YCbCr 422, single plane
+ - Plane 0: 3 components
+ * Component 0: Y(8)
+ * Component 1: Cb(8, 2x1 subsampled)
+ * Component 2: Cr(8, 2x1 subsampled)
+
+ * - DRM_FORMAT_Y210
+ - 10-bit per component YCbCr 422, single plane
+ - Plane 0: 3 components
+ * Component 0: Y(10)
+ * Component 1: Cb(10, 2x1 subsampled)
+ * Component 2: Cr(10, 2x1 subsampled)
+
+ * - DRM_FORMAT_YUV420_8BIT
+ - 8-bit per component YCbCr 420, single plane
+ - Plane 0: 3 components
+ * Component 0: Y(8)
+ * Component 1: Cb(8, 2x2 subsampled)
+ * Component 2: Cr(8, 2x2 subsampled)
+
+ * - DRM_FORMAT_YUV420_10BIT
+ - 10-bit per component YCbCr 420, single plane
+ - Plane 0: 3 components
+ * Component 0: Y(10)
+ * Component 1: Cb(10, 2x2 subsampled)
+ * Component 2: Cr(10, 2x2 subsampled)
+
+ * - DRM_FORMAT_NV12
+ - 8-bit per component YCbCr 420, two plane
+ - Plane 0: 1 component
+ * Component 0: Y(8)
+ Plane 1: 2 components
+ * Component 0: Cb(8, 2x2 subsampled)
+ * Component 1: Cr(8, 2x2 subsampled)
+
+ * - DRM_FORMAT_P010
+ - 10-bit per component YCbCr 420, two plane
+ - Plane 0: 1 component
+ * Component 0: Y(10)
+ Plane 1: 2 components
+ * Component 0: Cb(10, 2x2 subsampled)
+ * Component 1: Cr(10, 2x2 subsampled)
+
+ * - DRM_FORMAT_YUV420
+ - 8-bit per component YCbCr 420, three plane
+ - Plane 0: 1 component
+ * Component 0: Y(8)
+ Plane 1: 1 component
+ * Component 0: Cb(8, 2x2 subsampled)
+ Plane 2: 1 component
+ * Component 0: Cr(8, 2x2 subsampled)
diff --git a/Documentation/gpu/drivers.rst b/Documentation/gpu/drivers.rst
index d3ab6abae838..304691cb90e8 100644
--- a/Documentation/gpu/drivers.rst
+++ b/Documentation/gpu/drivers.rst
@@ -13,6 +13,7 @@ GPU Driver Documentation
vc4
bridge/dw-hdmi
xen-front
+ afbc

.. only:: subproject and html

diff --git a/MAINTAINERS b/MAINTAINERS
index 4af7f6119530..8fb92f162ae1 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1036,6 +1036,7 @@ M: Mali DP Maintainers <malidp@foss.arm.com>
S: Supported
F: drivers/gpu/drm/arm/
F: Documentation/devicetree/bindings/display/arm,malidp.txt
+F: Documentation/gpu/afbc.rst

ARM MFM AND FLOPPY DRIVERS
M: Ian Molton <spyro@f2s.com>
diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index 41f4610e471c..4ef685a02adc 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -494,6 +494,9 @@ extern "C" {
* AFBC has several features which may be supported and/or used, which are
* represented using bits in the modifier. Not all combinations are valid,
* and different devices or use-cases may support different combinations.
+ *
+ * Further information on the use of AFBC modifiers can be found in
+ * Documentation/gpu/afbc.rst
*/
#define DRM_FORMAT_MOD_ARM_AFBC(__afbc_mode) fourcc_mod_code(ARM, __afbc_mode)

--
2.16.1