[PATCH v8 4/8] media: Documentation: uapi: Add V4L2 ISP documentation

[Jacopo Mondi <jacopo.mondi@ideasonboard.com>]

Add userspace documentation for V4L2 ISP generic parameters and
statistics formats.

Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Michael Riesch <michael.riesch@collabora.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
---
 .../userspace-api/media/v4l/meta-formats.rst       |   1 +
 Documentation/userspace-api/media/v4l/v4l2-isp.rst | 120 +++++++++++++++++++++
 MAINTAINERS                                        |   1 +
 3 files changed, 122 insertions(+)

diff --git a/Documentation/userspace-api/media/v4l/meta-formats.rst b/Documentation/userspace-api/media/v4l/meta-formats.rst
index d9868ee88a0717c1acaa4ee477eaed96a6411f73..7b758ea9eb4ac3c4b354bf8e2f319985ed9e2b37 100644
--- a/Documentation/userspace-api/media/v4l/meta-formats.rst
+++ b/Documentation/userspace-api/media/v4l/meta-formats.rst
@@ -25,3 +25,4 @@ These formats are used for the :ref:`metadata` interface only.
     metafmt-vivid
     metafmt-vsp1-hgo
     metafmt-vsp1-hgt
+    v4l2-isp
diff --git a/Documentation/userspace-api/media/v4l/v4l2-isp.rst b/Documentation/userspace-api/media/v4l/v4l2-isp.rst
new file mode 100644
index 0000000000000000000000000000000000000000..b53df722ed29117c3827314e844fc4de61343f40
--- /dev/null
+++ b/Documentation/userspace-api/media/v4l/v4l2-isp.rst
@@ -0,0 +1,120 @@
+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _v4l2-isp:
+
+************************
+Generic V4L2 ISP formats
+************************
+
+ISP configuration and statistics: theory of operations
+======================================================
+
+ISP configuration parameters are computed by userspace and programmed into a
+*parameters buffer* which is queued to the ISP driver on a per-frame basis.
+
+ISP statistics are collected at a specific time point and drivers use them to
+populate a *statistics buffer* which is then returned to userspace.
+
+The parameters and statistics buffers are organized in a driver-specific
+way, and their data layout differs between one driver and another.
+
+ISP drivers generally exchange parameters and statistics with userspace through
+a metadata output and capture node respectively, implementing the
+:c:type:`v4l2_meta_format` interface. Each ISP driver defines one metadata
+capture format and one metadata output format to be used on those video nodes,
+and the buffer content layout and organization is fixed by the format definition.
+
+The uAPI/ABI problem
+--------------------
+
+By upstreaming the metadata formats that describe the parameters and statistics
+buffers layout, driver developers make them part of the Linux kernel ABI. As for
+most peripherals, ISP driver development in Linux is often an iterative process,
+in which not all of the hardware features are supported in the first version.
+
+The support for new features and/or bug fixes may land in the kernel at a later
+stage and require changes to the metadata formats definition. This is
+considered an ABI breakage that is strictly forbidden by the Linux kernel
+policies. For this reason, any change in the ISP parameters and statistics
+buffer layout would require defining a new metadata format.
+
+For these reasons Video4Linux2 has introduced support for generic ISP parameters
+and statistics data types, designed with the goal of being:
+
+- Extensible: new features can be added later on without breaking the existing
+  interface
+- Versioned: different versions of the format can be defined without
+  breaking the existing interface
+
+ISP configuration
+=================
+
+Before the introduction of generic formats
+------------------------------------------
+
+Metadata output formats that describe ISP configuration parameters were
+typically realized by defining C structures that reflect the ISP registers
+layout and get populated by userspace before queueing the buffer to the ISP.
+Each C structure usually corresponds to one ISP *processing block*, with each
+block implementing one of the ISP supported features.
+
+The number of supported ISP blocks, the layout of their configuration data are
+fixed by the format definition, incurring in the above described uAPI/uABI
+problem.
+
+Generic ISP parameters
+----------------------
+
+The generic ISP configuration parameters format is realized by a defining a
+single C structure that contains a header, followed by a binary buffer where
+userspace programs a variable number of ISP configuration data block, one for
+each supported ISP feature.
+
+The :c:type:`v4l2_isp_params_buffer` structure defines the parameters buffer
+header which is followed by a binary buffer of ISP configuration parameters.
+Userspace shall correctly populate the buffer header with the versioning
+information and with the size (in bytes) of the binary data buffer where it will
+store the ISP blocks configuration.
+
+Each *ISP configuration block* is preceded by an header implemented by the
+:c:type:`v4l2_isp_params_block_header` structure, followed by the configuration
+parameters for that specific block, defined by the ISP driver specific data
+types.
+
+Userspace applications are responsible for correctly populating each block's
+header fields (type, flags and size) and the block-specific parameters.
+
+ISP Block enabling, disabling and configuration
+-----------------------------------------------
+
+When userspace wants to configure and enable an ISP block it shall fully
+populate the block configuration and set the V4L2_ISP_PARAMS_FL_BLOCK_ENABLE
+bit in the block header's `flags` field.
+
+When userspace simply wants to disable an ISP block the
+V4L2_ISP_PARAMS_FL_BLOCK_DISABLE bit should be set in block header's `flags`
+field. Drivers accept a configuration parameters block with no additional
+data after the header in this case.
+
+If the configuration of an already active ISP block has to be updated,
+userspace shall fully populate the ISP block parameters and omit setting the
+V4L2_ISP_PARAMS_FL_BLOCK_ENABLE and V4L2_ISP_PARAMS_FL_BLOCK_DISABLE bits in the
+header's `flags` field.
+
+Setting both the V4L2_ISP_PARAMS_FL_BLOCK_ENABLE and
+V4L2_ISP_PARAMS_FL_BLOCK_DISABLE bits in the flags field is not allowed and not
+accepted.
+
+Any further extension to the parameters layout that happens after the ISP driver
+has been merged in Linux can be implemented by adding new blocks definition
+without invalidating the existing ones.
+
+ISP statistics
+==============
+
+Support for generic statistics format is not yet implemented in Video4Linux2.
+
+V4L2 ISP uAPI data types
+========================
+
+.. kernel-doc:: include/uapi/linux/media/v4l2-isp.h
diff --git a/MAINTAINERS b/MAINTAINERS
index d925745077f21e5a1388a30217a24beeb4fff3b5..f52237d57710cadff78b297d2b4610b508f55092 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -26856,6 +26856,7 @@ V4L2 GENERIC ISP PARAMETERS AND STATISTIC FORMATS
 M:	Jacopo Mondi <jacopo.mondi@ideasonboard.com>
 L:	linux-media@vger.kernel.org
 S:	Maintained
+F:	Documentation/userspace-api/media/v4l/v4l2-isp.rst
 F:	include/uapi/linux/media/v4l2-isp.h
 
 VF610 NAND DRIVER

-- 
2.51.0


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