[libcamera-devel] [PATCH v2] Documentation: IPU3 IPA Design guide

[Jean-Michel Hautbois]

Hi Kieran,

On 16/09/2021 14:51, Kieran Bingham wrote:
> The IPU3 IPA implements the basic 3A using the ImgU ISP.
> 
> Provide an overview document to describe its operations, and provide a
> block diagram to help visualise how the components are put together to
> assist any new developers exploring the code.
> 
> Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> Reviewed-by: Umang Jain <umang.jain at ideasonboard.com>
> 
> ---
> v2:
>  - /accelerator cluster/processing block/ (and refactor)
>  - /Pipeline/pipeline/
>  - /Camera Sensor/camera sensor/
>  - /CPU accessible/CPU-accessible/
>  - Remove updated control parameters from IPASessionConfiguration
>  - Expand pre-frame preparation to match post-frame with the event
>    descriptions.
>  - Add Sensor Controls brief
>  - Move to src/ipa/ipu3/
>  - Lower indentation of the block diagrams (keep under 80chars)
>  - reference mapBuffers() call for passing buffers in
>  - reference unmapBuffers() after stop()
> 
>  src/ipa/ipu3/ipu3-ipa-design-guide.rst | 155 +++++++++++++++++++++++++
>  1 file changed, 155 insertions(+)
>  create mode 100644 src/ipa/ipu3/ipu3-ipa-design-guide.rst
> 
> diff --git a/src/ipa/ipu3/ipu3-ipa-design-guide.rst b/src/ipa/ipu3/ipu3-ipa-design-guide.rst
> new file mode 100644
> index 000000000000..e5ee508d62e9
> --- /dev/null
> +++ b/src/ipa/ipu3/ipu3-ipa-design-guide.rst
> @@ -0,0 +1,155 @@
> +IPU3 IPA Architecture Design and Overview
> +=========================================
> +
> +The IPU3 IPA is built as a modular and extensible framework with an
> +upper layer to manage the interactions with the pipeline handler, and
> +the image processing algorithms split to compartmentalise the processing
> +required for each processing block, making use of the fixed-function
> +accelerators provided by the ImgU ISP.

I am not native, and it might very well explain it but... isn't that
sentence very long ? I read it multiple times to have it all... :-/

> +
> +The core IPU3 class is responsible for initialisation and construction
> +of the algorithm components, processing controls set by the requests
> +from applications, and managing events from the pipeline handler.
> +
> +::
> +
> +      ┌───────────────────────────────────────────┐
> +      │      IPU3 Pipeline Handler                │
> +      │   ┌────────┐    ┌────────┐    ┌────────┐  │
> +      │   │        │    │        │    │        │  │
> +      │   │ Sensor ├───►│  CIO2  ├───►│  ImgU  ├──►
> +      │   │        │    │        │    │        │  │
> +      │   └────────┘    └────────┘    └─▲────┬─┘  │    P: Parameter Buffer
> +      │                                 │P   │    │    S: Statistics Buffer
> +      │                                 │    │S   │
> +      └─┬───┬───┬──────┬────┬────┬────┬─┴────▼─┬──┘    1: init()
> +        │   │   │      │ ▲  │ ▲  │ ▲  │ ▲      │       2: configure()
> +        │1  │2  │3     │4│  │4│  │4│  │4│      │5      3: mapBuffers(), start()
> +        ▼   ▼   ▼      ▼ │  ▼ │  ▼ │  ▼ │      ▼       4: processEvent()
> +      ┌──────────────────┴────┴────┴────┴─────────┐    5: stop(), unmapBuffers()
> +      │ IPU3 IPA                                  │
> +      │                 ┌───────────────────────┐ │
> +      │ ┌───────────┐   │ Algorithms            │ │
> +      │ │IPAContext │   │          ┌─────────┐  │ │
> +      │ │ ┌───────┐ │   │          │ ...     │  │ │
> +      │ │ │       │ │   │        ┌─┴───────┐ │  │ │
> +      │ │ │  SC   │ │   │        │ Tonemap ├─┘  │ │
> +      │ │ │       │ ◄───►      ┌─┴───────┐ │    │ │
> +      │ │ ├───────┤ │   │      │ AWB     ├─┘    │ │
> +      │ │ │       │ │   │    ┌─┴───────┐ │      │ │
> +      │ │ │  FC   │ │   │    │ AGC     ├─┘      │ │
> +      │ │ │       │ │   │    │         │        │ │
> +      │ │ └───────┘ │   │    └─────────┘        │ │
> +      │ └───────────┘   └───────────────────────┘ │
> +      └───────────────────────────────────────────┘
> +        SC: IPASessionConfiguration
> +        FC: IPAFrameContext(s)
> +
> +The IPA instance is constructed and initialised at the point a Camera is
> +created by the IPU3 pipeline handler. The initialisation call provides
> +details about which camera sensor is being used, and the controls that
> +it has available, along with their default values and ranges.
> +
> +Buffers
> +~~~~~~~
> +
> +The IPA will have Parameter and Statistics buffers shared with it from
> +the IPU3 Pipeline handler. These buffers will be passed to the IPA using
> +the ``mapBuffers()`` call before the ``start()`` operation occurs.
> +
> +The IPA will map the buffers into CPU-accessible memory, associated with
> +a buffer ID, and further events for sending or receiving parameter and
> +statistics buffers will reference the ID to avoid expensive memory
> +mapping operations, or the passing of file handles during streaming.
> +
> +After the ``stop()`` operation occurs, these buffers will be unmapped
> +when requested by the pipeline handler using the ``unmapBuffers()`` call
> +and no further access to the buffers is permitted.
> +
> +Context
> +~~~~~~~
> +
> +Algorithm calls will always have the ``IPAContext`` available to them.
> +This context comprises of two parts:
> +
> +-  IPA Session Configuration
> +-  IPA Frame Context
> +
> +The session configuration structure ``IPASessionConfiguration``
> +represents constant parameters determined before streaming commenced
> +during ``configure()``.
> +
> +The IPA Frame Context provides the storage for algorithms for a single
> +frame operation.
> +
> +The ``IPAFrameContext`` structure may be extended to an array, list, or
> +queue to store historical state for each frame, allowing algorithms to
> +obtain and reference results of calculations which are deeply pipelined.
> +This may only be done if an algorithm needs to know the context that was
> +applied at the frame the statistics were produced for, rather than the
> +previous or current frame.
> +
> +Presently there is a single ``IPAFrameContext`` without historical data,
> +and the context is maintained and updated through successive processing
> +operations.
> +
> +Operating
> +~~~~~~~~~
> +
> +There are three main interactions with the algorithms for the IPU3 IPA
> +to operate when running:
> +
> +-  configure()
> +-  processEvent(``EventFillParams``)
> +-  processEvent(``EventStatReady``)
> +
> +The configuration phase allows the pipeline-handler to inform the IPA of
> +the current stream configurations, which is then passed into each
> +algorithm to provide an opportunity to identify and track state of the
> +hardware, such as image size or ImgU pipeline configurations.
> +
> +Pre-frame preparation
> +~~~~~~~~~~~~~~~~~~~~~
> +
> +When configured, the IPA is notified by the pipeline handler of the
> +Camera ``start()`` event, after which incoming requests will be queued
> +for processing, requiring a parameter buffer (``ipu3_uapi_params``) to
> +be populated for the ImgU. This is given to the IPA through the
> +``EventFillParams`` event, and then passed directly to each algorithm
> +through the ``prepare()`` call allowing the ISP configuration to be
> +updated for the needs of each component that the algorithm is
> +responsible for.
> +
> +The algorithm should set the use flag (``ipu3_uapi_flags``) for any
> +structure that it modifies, and it should take care to ensure that any
> +structure set by a use flag is fully initialised to suitable values.
> +
> +The parameter buffer is returned to the pipeline handler through the
> +``ActionParamFilled`` event, and from there queued to the ImgU along
> +with a raw frame captured with the CIO2.
> +
> +Post-frame completion
> +~~~~~~~~~~~~~~~~~~~~~
> +
> +When the capture of an image is completed, and successfully processed
> +through the ImgU, the generated statistics buffer
> +(``ipu3_uapi_stats_3a``) is given to the IPA through the
> +``EventStatReady`` event. This provides the IPA with an opportunity to
> +examine the results of the ISP and run the calculations required by each
> +algorithm on the new data. The algorithms may require context from the
> +operations of other algorithms, for example, the AWB might choose to use
> +a scene brightness determined by the AGC. It is important that the
> +algorithms are ordered to ensure that required results are determined
> +before they are needed.
> +
> +The ordering of the algorithm processing is determined by their
> +placement in the ``IPU3::algorithms_`` ordered list.
> +
> +Sensor Controls
> +~~~~~~~~~~~~~~~
> +
> +The AutoExposure and AutoGain (AGC) algorithm differs slightly from the
> +others as it requires operating directly on the sensor, as opposed to
> +through the ImgU ISP. To support this, there is a dedicated action
> +`ActionSetSensorControls` to allow the IPA to request controls to be set
> +on the camera sensor through the pipeline handler.
> \ No newline at end of file

Where is this "No newline" coming from ?

Apart from that:
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>