[PATCH v10 08/17] media: Documentation: Add Mali-C55 ISP Documentation

Daniel Scally
2025-06-24T10:21:47Z

Add a documentation page for the mali-c55 driver, which gives a brief
overview of the hardware and explains how to use the driver's capture
devices and the crop/scaler functions.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Nayden Kanchev <nayden.kanchev@arm.com>
Co-developed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
---
Changes in v10:

- None

Changes in v9:

- None

Changes in v8:

- None

Changes in v7:

- None

Changes in v6:

- Minor formatting changes

Changes in v5:

- None

Changes in v4:
- None

Changes in v3:
- Documented the synchronised buffer sequence numbers (Sakari)
- Clarified that the downscale pipe cannot output raw data, the ISP'S
resolution limits and choice of media bus format code (Kieran)

Changes in v2:

- none
---
Documentation/admin-guide/media/mali-c55-graph.dot | 19 ++
Documentation/admin-guide/media/mali-c55.rst | 340 +++++++++++++++++++++
Documentation/admin-guide/media/v4l-drivers.rst | 1 +
3 files changed, 360 insertions(+)

diff --git a/Documentation/admin-guide/media/mali-c55-graph.dot b/Documentation/admin-guide/media/mali-c55-graph.dot
new file mode 100644
index 0000000000000000000000000000000000000000..0775ba42bf4cfacc3911dc96b3c20a687bd25e4b
--- /dev/null
+++ b/Documentation/admin-guide/media/mali-c55-graph.dot
@@ -0,0 +1,19 @@
+digraph board {
+ rankdir=TB
+ n00000001 [label="{{} | mali-c55 tpg\n/dev/v4l-subdev0 | {<port0> 0}}", shape=Mrecord, style=filled, fillcolor=green]
+ n00000001:port0 -> n00000003:port0 [style=dashed]
+ n00000003 [label="{{<port0> 0} | mali-c55 isp\n/dev/v4l-subdev1 | {<port1> 1 | <port2> 2}}", shape=Mrecord, style=filled, fillcolor=green]
+ n00000003:port1 -> n00000007:port0 [style=bold]
+ n00000003:port2 -> n00000007:port2 [style=bold]
+ n00000003:port1 -> n0000000b:port0 [style=bold]
+ n00000007 [label="{{<port0> 0 | <port2> 2} | mali-c55 resizer fr\n/dev/v4l-subdev2 | {<port1> 1}}", shape=Mrecord, style=filled, fillcolor=green]
+ n00000007:port1 -> n0000000e [style=bold]
+ n0000000b [label="{{<port0> 0} | mali-c55 resizer ds\n/dev/v4l-subdev3 | {<port1> 1}}", shape=Mrecord, style=filled, fillcolor=green]
+ n0000000b:port1 -> n00000012 [style=bold]
+ n0000000e [label="mali-c55 fr\n/dev/video0", shape=box, style=filled, fillcolor=yellow]
+ n00000012 [label="mali-c55 ds\n/dev/video1", shape=box, style=filled, fillcolor=yellow]
+ n00000022 [label="{{<port0> 0} | csi2-rx\n/dev/v4l-subdev4 | {<port1> 1}}", shape=Mrecord, style=filled, fillcolor=green]
+ n00000022:port1 -> n00000003:port0
+ n00000027 [label="{{} | imx415 1-001a\n/dev/v4l-subdev5 | {<port0> 0}}", shape=Mrecord, style=filled, fillcolor=green]
+ n00000027:port0 -> n00000022:port0 [style=bold]
+}
\ No newline at end of file
diff --git a/Documentation/admin-guide/media/mali-c55.rst b/Documentation/admin-guide/media/mali-c55.rst
new file mode 100644
index 0000000000000000000000000000000000000000..72cdded507b3a7918e28d9bd9a1c6f72ddf9e664
--- /dev/null
+++ b/Documentation/admin-guide/media/mali-c55.rst
@@ -0,0 +1,340 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================================
+ARM Mali-C55 Image Signal Processor driver
+==========================================
+
+Introduction
+============
+
+This file documents the driver for ARM's Mali-C55 Image Signal Processor. The
+driver is located under drivers/media/platform/arm/mali-c55.
+
+The Mali-C55 ISP receives data in either raw Bayer format or RGB/YUV format from
+sensors through either a parallel interface or a memory bus before processing it
+and outputting it through an internal DMA engine. Two output pipelines are
+possible (though one may not be fitted, depending on the implementation). These
+are referred to as "Full resolution" and "Downscale", but the naming is historic
+and both pipes are capable of cropping/scaling operations. The full resolution
+pipe is also capable of outputting RAW data, bypassing much of the ISP's
+processing. The downscale pipe cannot output RAW data. An integrated test
+pattern generator can be used to drive the ISP and produce image data in the
+absence of a connected camera sensor. The driver module is named mali_c55, and
+is enabled through the CONFIG_VIDEO_MALI_C55 config option.
+
+The driver implements V4L2, Media Controller and V4L2 Subdevice interfaces and
+expects camera sensors connected to the ISP to have V4L2 subdevice interfaces.
+
+Mali-C55 ISP hardware
+=====================
+
+A high level functional view of the Mali-C55 ISP is presented below. The ISP
+takes input from either a live source or through a DMA engine for memory input,
+depending on the SoC integration.::
+
+ +---------+ +----------+ +--------+
+ | Sensor |--->| CSI-2 Rx | "Full Resolution" | DMA |
+ +---------+ +----------+ |\ Output +--->| Writer |
+ | | \ | +--------+
+ | | \ +----------+ +------+---> Streaming I/O
+ +------------+ +------->| | | | |
+ | | | |-->| Mali-C55 |--+
+ | DMA Reader |--------------->| | | ISP | |
+ | | | / | | | +---> Streaming I/O
+ +------------+ | / +----------+ | |
+ |/ +------+
+ | +--------+
+ +--->| DMA |
+ "Downscaled" | Writer |
+ Output +--------+
+
+Media Controller Topology
+=========================
+
+An example of the ISP's topology (as implemented in a system with an IMX415
+camera sensor and generic CSI-2 receiver) is below:
+
+
+.. kernel-figure:: mali-c55-graph.dot
+ :alt: mali-c55-graph.dot
+ :align: center
+
+The driver has 4 V4L2 subdevices:
+
+- `mali_c55 isp`: Responsible for configuring input crop and color space
+ conversion
+- `mali_c55 tpg`: The test pattern generator, emulating a camera sensor.
+- `mali_c55 resizer fr`: The Full-Resolution pipe resizer
+- `mali_c55 resizer ds`: The Downscale pipe resizer
+
+The driver has 2 V4L2 video devices:
+
+- `mali-c55 fr`: The full-resolution pipe's capture device
+- `mali-c55 ds`: The downscale pipe's capture device
+
+Frame sequences are synchronised across to two capture devices, meaning if one
+pipe is started later than the other the sequence numbers returned in its
+buffers will match those of the other pipe rather than starting from zero.
+
+Idiosyncrasies
+--------------
+
+**mali-c55 isp**
+The `mali-c55 isp` subdevice has a single sink pad to which all sources of data
+should be connected. The active source is selected by enabling the appropriate
+media link and disabling all others. The ISP has two source pads, reflecting the
+different paths through which it can internally route data. Tap points within
+the ISP allow users to divert data to avoid processing by some or all of the
+hardware's processing steps. The diagram below is intended only to highlight how
+the bypassing works and is not a true reflection of those processing steps; for
+a high-level functional block diagram see ARM's developer page for the
+ISP [3]_::
+
+ +--------------------------------------------------------------+
+ | Possible Internal ISP Data Routes |
+ | +------------+ +----------+ +------------+ |
+ +---+ | | | | | Colour | +---+
+ | 0 |--+-->| Processing |->| Demosaic |->| Space |--->| 1 |
+ +---+ | | | | | | Conversion | +---+
+ | | +------------+ +----------+ +------------+ |
+ | | +---+
+ | +---------------------------------------------------| 2 |
+ | +---+
+ | |
+ +--------------------------------------------------------------+
+
+
+.. flat-table::
+ :header-rows: 1
+
+ * - Pad
+ - Direction
+ - Purpose
+
+ * - 0
+ - sink
+ - Data input, connected to the TPG and camera sensors
+
+ * - 1
+ - source
+ - RGB/YUV data, connected to the FR and DS V4L2 subdevices
+
+ * - 2
+ - source
+ - RAW bayer data, connected to the FR V4L2 subdevices
+
+The ISP is limited to both input and output resolutions between 640x480 and
+8192x8192, and this is reflected in the ISP and resizer subdevice's .set_fmt()
+operations.
+
+**mali-c55 resizer fr**
+The `mali-c55 resizer fr` subdevice has two _sink_ pads to reflect the different
+insertion points in the hardware (either RAW or demosaiced data):
+
+.. flat-table::
+ :header-rows: 1
+
+ * - Pad
+ - Direction
+ - Purpose
+
+ * - 0
+ - sink
+ - Data input connected to the ISP's demosaiced stream.
+
+ * - 1
+ - source
+ - Data output connected to the capture video device
+
+ * - 2
+ - sink
+ - Data input connected to the ISP's raw data stream
+
+The data source in use is selected through the routing API; two routes each of a
+single stream are available:
+
+.. flat-table::
+ :header-rows: 1
+
+ * - Sink Pad
+ - Source Pad
+ - Purpose
+
+ * - 0
+ - 1
+ - Demosaiced data route
+
+ * - 2
+ - 1
+ - Raw data route
+
+
+If the demosaiced route is active then the FR pipe is only capable of output
+in RGB/YUV formats. If the raw route is active then the output reflects the
+input (which may be either Bayer or RGB/YUV data).
+
+Using the driver to capture video
+=================================
+
+Using the media controller APIs we can configure the input source and ISP to
+capture images in a variety of formats. In the examples below, configuring the
+media graph is done with the v4l-utils [1]_ package's media-ctl utility.
+Capturing the images is done with yavta [2]_.
+
+Configuring the input source
+----------------------------
+
+The first step is to set the input source that we wish by enabling the correct
+media link. Using the example topology above, we can select the TPG as follows:
+
+.. code-block:: none
+
+ media-ctl -l "'lte-csi2-rx':1->'mali-c55 isp':0[0]"
+ media-ctl -l "'mali-c55 tpg':0->'mali-c55 isp':0[1]"
+
+Configuring which video devices will stream data
+------------------------------------------------
+
+The driver will wait for all video devices to have their VIDIOC_STREAMON ioctl
+called before it tells the sensor to start streaming. To facilitate this we need
+to enable links to the video devices that we want to use. In the example below
+we enable the links to both of the image capture video devices
+
+.. code-block:: none
+
+ media-ctl -l "'mali-c55 resizer fr':1->'mali-c55 fr':0[1]"
+ media-ctl -l "'mali-c55 resizer ds':1->'mali-c55 ds':0[1]"
+
+Capturing bayer data from the source and processing to RGB/YUV
+--------------------------------------------------------------
+
+To capture 1920x1080 bayer data from the source and push it through the ISP's
+full processing pipeline, we configure the data formats appropriately on the
+source, ISP and resizer subdevices and set the FR resizer's routing to select
+processed data. The media bus format on the resizer's source pad will be either
+RGB121212_1X36 or YUV10_1X30, depending on whether you want to capture RGB or
+YUV. The ISP's debayering block outputs RGB data natively, setting the source
+pad format to YUV10_1X30 enables the colour space conversion block.
+
+In this example we target RGB565 output, so select RGB121212_1X36 as the resizer
+source pad's format:
+
+.. code-block:: none
+
+ # Set formats on the TPG and ISP
+ media-ctl -V "'mali-c55 tpg':0[fmt:SRGGB20_1X20/1920x1080]"
+ media-ctl -V "'mali-c55 isp':0[fmt:SRGGB20_1X20/1920x1080]"
+ media-ctl -V "'mali-c55 isp':1[fmt:SRGGB20_1X20/1920x1080]"
+
+ # Set routing on the FR resizer
+ media-ctl -R "'mali-c55 resizer fr'[0/0->1/0[1],2/0->1/0[0]]"
+
+ # Set format on the resizer, must be done AFTER the routing.
+ media-ctl -V "'mali-c55 resizer fr':1[fmt:RGB121212_1X36/1920x1080]"
+
+The downscale output can also be used to stream data at the same time. In this
+case since only processed data can be captured through the downscale output no
+routing need be set:
+
+.. code-block:: none
+
+ # Set format on the resizer
+ media-ctl -V "'mali-c55 resizer ds':1[fmt:RGB121212_1X36/1920x1080]"
+
+Following which images can be captured from both the FR and DS output's video
+devices (simultaneously, if desired):
+
+.. code-block:: none
+
+ yavta -f RGB565 -s 1920x1080 -c10 /dev/video0
+ yavta -f RGB565 -s 1920x1080 -c10 /dev/video1
+
+Cropping the image
+~~~~~~~~~~~~~~~~~~
+
+Both the full resolution and downscale pipes can crop to a minimum resolution of
+640x480. To crop the image simply configure the resizer's sink pad's crop and
+compose rectangles and set the format on the video device:
+
+.. code-block:: none
+
+ media-ctl -V "'mali-c55 resizer fr':0[fmt:RGB121212_1X36/1920x1080 crop:(480,270)/640x480 compose:(0,0)/640x480]"
+ media-ctl -V "'mali-c55 resizer fr':1[fmt:RGB121212_1X36/640x480]"
+ yavta -f RGB565 -s 640x480 -c10 /dev/video0
+
+Downscaling the image
+~~~~~~~~~~~~~~~~~~~~~
+
+Both the full resolution and downscale pipes can downscale the image by up to 8x
+provided the minimum 640x480 output resolution is adhered to. For the best image
+result the scaling ratio for each direction should be the same. To configure
+scaling we use the compose rectangle on the resizer's sink pad:
+
+.. code-block:: none
+
+ media-ctl -V "'mali-c55 resizer fr':0[fmt:RGB121212_1X36/1920x1080 crop:(0,0)/1920x1080 compose:(0,0)/640x480]"
+ media-ctl -V "'mali-c55 resizer fr':1[fmt:RGB121212_1X36/640x480]"
+ yavta -f RGB565 -s 640x480 -c10 /dev/video0
+
+Capturing images in YUV formats
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If we need to output YUV data rather than RGB the color space conversion block
+needs to be active, which is achieved by setting MEDIA_BUS_FMT_YUV10_1X30 on the
+resizer's source pad. We can then configure a capture format like NV12 (here in
+its multi-planar variant)
+
+.. code-block:: none
+
+ media-ctl -V "'mali-c55 resizer fr':1[fmt:YUV10_1X30/1920x1080]"
+ yavta -f NV12M -s 1920x1080 -c10 /dev/video0
+
+Capturing RGB data from the source and processing it with the resizers
+----------------------------------------------------------------------
+
+The Mali-C55 ISP can work with sensors capable of outputting RGB data. In this
+case although none of the image quality blocks would be used it can still
+crop/scale the data in the usual way. For this reason RGB data input to the ISP
+still goes through the ISP subdevice's pad 1 to the resizer.
+
+To achieve this, the ISP's sink pad's format is set to
+MEDIA_BUS_FMT_RGB202020_1X60 - this reflects the format that data must be in to
+work with the ISP. Converting the camera sensor's output to that format is the
+responsibility of external hardware.
+
+In this example we ask the test pattern generator to give us RGB data instead of
+bayer.
+
+.. code-block:: none
+
+ media-ctl -V "'mali-c55 tpg':0[fmt:RGB202020_1X60/1920x1080]"
+ media-ctl -V "'mali-c55 isp':0[fmt:RGB202020_1X60/1920x1080]"
+
+Cropping or scaling the data can be done in exactly the same way as outlined
+earlier.
+
+Capturing raw data from the source and outputting it unmodified
+-----------------------------------------------------------------
+
+The ISP can additionally capture raw data from the source and output it on the
+full resolution pipe only, completely unmodified. In this case the downscale
+pipe can still process the data normally and be used at the same time.
+
+To configure raw bypass the FR resizer's subdevice's routing table needs to be
+configured, followed by formats in the appropriate places:
+
+.. code-block:: none
+
+ media-ctl -R "'mali-c55 resizer fr'[0/0->1/0[0],2/0->1/0[1]]"
+ media-ctl -V "'mali-c55 isp':0[fmt:RGB202020_1X60/1920x1080]"
+ media-ctl -V "'mali-c55 resizer fr':2[fmt:RGB202020_1X60/1920x1080]"
+ media-ctl -V "'mali-c55 resizer fr':1[fmt:RGB202020_1X60/1920x1080]"
+
+ # Set format on the video device and stream
+ yavta -f RGB565 -s 1920x1080 -c10 /dev/video0
+
+References
+==========
+.. [1] https://git.linuxtv.org/v4l-utils.git/
+.. [2] https://git.ideasonboard.org/yavta.git
+.. [3] https://developer.arm.com/Processors/Mali-C55
diff --git a/Documentation/admin-guide/media/v4l-drivers.rst b/Documentation/admin-guide/media/v4l-drivers.rst
index 3bac5165b13416ee94d9f98ccc22d07d334ae90e..ebd9b15efa903ecb62869d4b7e0b0d6f3ed73589 100644
--- a/Documentation/admin-guide/media/v4l-drivers.rst
+++ b/Documentation/admin-guide/media/v4l-drivers.rst
@@ -19,6 +19,7 @@ Video4Linux (V4L) driver-specific documentation
ipu3
ipu6-isys
ivtv
+ mali-c55
mgb4
omap3isp
philips

--
2.34.1