[libcamera-devel] [PATCH v3 11/19] Documentation: IPU3 IPA Design guide

[Jean-Michel Hautbois]

Hi,

On 25/10/2021 23:30, Laurent Pinchart wrote:
> Hi Jean-Michel and Kieran,
> 
> Thank you for the patch.
> 
> On Fri, Oct 22, 2021 at 05:12:10PM +0200, Jean-Michel Hautbois wrote:
>> From: Kieran Bingham <kieran.bingham at ideasonboard.com>
>>
>> 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 00000000..e5ee508d
>> --- /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.
>> +
>> +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
> 
> Maybe a newline at end of file then ?
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> 

Added, and at the same time:
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>