.. SPDX-License-Identifier: GPL-2.0+

===================================
 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_NV16
     - 8-bit per component YCbCr 422, two plane
     - Plane 0: 1 component
              * Component 0: Y(8)
       Plane 1: 2 components
              * Component 0: Cb(8, 2x1 subsampled)
              * Component 1: 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_P210
     - 10-bit per component YCbCr 422, two plane
     - Plane 0: 1 component
              * Component 0: Y(10)
       Plane 1: 2 components
              * Component 0: Cb(10, 2x1 subsampled)
              * Component 1: 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)