media: uapi: Document PiSP Compressed RAW Bayer formats

Document the Raspberry Pi compressed RAW Bayer formats.

The compression algorithm description is provided by Nick Hollinghurst
<nick.hollinghurst@raspberrypi.com> from Raspberry Pi.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>

2 files changed, 75 insertions, 0 deletions

diff --git a/Documentation/userspace-api/media/v4l/pixfmt-bayer.rst b/Documentation/userspace-api/media/v4l/pixfmt-bayer.rst
index 2500413e5f43..ed3eb432967d 100644
--- a/Documentation/userspace-api/media/v4l/pixfmt-bayer.rst
+++ b/Documentation/userspace-api/media/v4l/pixfmt-bayer.rst
@@ -20,6 +20,7 @@ orders. See also `the Wikipedia article on Bayer filter
     :maxdepth: 1
 
     pixfmt-srggb8
+    pixfmt-srggb8-pisp-comp
     pixfmt-srggb10
     pixfmt-srggb10p
     pixfmt-srggb10alaw8
diff --git a/Documentation/userspace-api/media/v4l/pixfmt-srggb8-pisp-comp.rst b/Documentation/userspace-api/media/v4l/pixfmt-srggb8-pisp-comp.rst
new file mode 100644
index 000000000000..5a82a15559d6
--- /dev/null
+++ b/Documentation/userspace-api/media/v4l/pixfmt-srggb8-pisp-comp.rst
@@ -0,0 +1,74 @@
+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _v4l2-pix-fmt-pisp-comp1-rggb:
+.. _v4l2-pix-fmt-pisp-comp1-grbg:
+.. _v4l2-pix-fmt-pisp-comp1-gbrg:
+.. _v4l2-pix-fmt-pisp-comp1-bggr:
+.. _v4l2-pix-fmt-pisp-comp1-mono:
+.. _v4l2-pix-fmt-pisp-comp2-rggb:
+.. _v4l2-pix-fmt-pisp-comp2-grbg:
+.. _v4l2-pix-fmt-pisp-comp2-gbrg:
+.. _v4l2-pix-fmt-pisp-comp2-bggr:
+.. _v4l2-pix-fmt-pisp-comp2-mono:
+
+**************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
+V4L2_PIX_FMT_PISP_COMP1_RGGB ('PC1R'), V4L2_PIX_FMT_PISP_COMP1_GRBG ('PC1G'), V4L2_PIX_FMT_PISP_COMP1_GBRG ('PC1g'), V4L2_PIX_FMT_PISP_COMP1_BGGR ('PC1B), V4L2_PIX_FMT_PISP_COMP1_MONO ('PC1M'), V4L2_PIX_FMT_PISP_COMP2_RGGB ('PC2R'), V4L2_PIX_FMT_PISP_COMP2_GRBG ('PC2G'), V4L2_PIX_FMT_PISP_COMP2_GBRG ('PC2g'), V4L2_PIX_FMT_PISP_COMP2_BGGR ('PC2B), V4L2_PIX_FMT_PISP_COMP2_MONO ('PC2M')
+**************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
+
+================================================
+Raspberry Pi PiSP compressed 8-bit Bayer formats
+================================================
+
+Description
+===========
+
+The Raspberry Pi ISP (PiSP) uses a family of three fixed-rate compressed Bayer
+formats. A black-level offset may be subtracted to improve compression
+efficiency; the nominal black level and amount of offset must be signalled out
+of band. Each scanline is padded to a multiple of 8 pixels wide, and each block
+of 8 horizontally-contiguous pixels is coded using 8 bytes.
+
+Mode 1 uses a quantization and delta-based coding scheme which preserves up to
+12 significant bits. Mode 2 is a simple sqrt-like companding scheme with 6 PWL
+chords, preserving up to 12 significant bits. Mode 3 combines both companding
+(with 4 chords) and the delta scheme, preserving up to 14 significant bits.
+
+The remainder of this description applies to Modes 1 and 3.
+
+Each block of 8 pixels is separated into even and odd phases of 4 pixels,
+coded independently by 32-bit words at successive locations in memory.
+The two LS bits of each 32-bit word give its "quantization mode".
+
+In quantization mode 0, the lowest 321 quantization levels are multiples of
+FSD/4096 and the remaining levels are successive multiples of FSD/2048.
+Quantization modes 1 and 2 use linear quantization with step sizes of
+FSD/1024 and FSD/512 respectively. Each of the four pixels is quantized
+independently, with rounding to the nearest level.
+In quantization mode 2 where the middle two samples have quantized values
+(q1,q2) both in the range [384..511], they are coded using 9 bits for q1
+followed by 7 bits for (q2 & 127). Otherwise, for quantization modes
+0, 1 and 2: a 9-bit field encodes MIN(q1,q2) which must be in the range
+[0..511] and a 7-bit field encodes (q2-q1+64) which must be in [0..127].
+
+Each of the outer samples (q0,q3) is encoded using a 7-bit field based
+on its inner neighbour q1 or q2. In quantization mode 2 where the inner
+sample has a quantized value in the range [448..511], the field value is
+(q0-384). Otherwise for quantization modes 0, 1 and 2: The outer sample
+is encoded as (q0-MAX(0,q1-64)). q3 is likewise coded based on q2.
+Each of these values must be in the range [0..127]. All these fields
+of 2, 9, 7, 7, 7 bits respectively are packed in little-endian order
+to give a 32-bit word with LE byte order.
+
+Quantization mode 3 has a "7.5-bit" escape, used when none of the above
+encodings will fit. Each pixel value is quantized to the nearest of 176
+levels, where the lowest 95 levels are multiples of FSD/256 and the
+remaining levels are multiples of FSD/128 (level 175 represents values
+very close to FSD and may require saturating arithmetic to decode).
+
+Each pair of quantized pixels (q0,q1) or (q2,q3) is jointly coded
+by a 15-bit field: 2816*(q0>>4) + 16*q1 + (q0&15).
+Three fields of 2, 15, 15 bits are packed in LE order {15,15,2}.
+
+An implementation of a software decoder of compressed formats is available
+in `Raspberry Pi camera applications code base
+<https://github.com/raspberrypi/rpicam-apps/blob/main/image/dng.cpp>`_.