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

[Laurent Pinchart]

Hi Kieran,

Thank you for the patch.

On Sun, Sep 12, 2021 at 10:30:17PM +0100, Kieran Bingham wrote:
> The IPU3 IPA implements the basic 3a using the ImgU ISP.

s/3a/3A/

Technically speaking we're only implementing 2 out of 3 at this point
:-)

> Provide an overview document to describe it's operations, and provide a

s/it's/its/

> 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>
> 
> ---
> This is really just an RFC: Particularly:
> 
>  - Should this content actually live in Documentation?
>    or is it more suited to a page under src/ipa/ipu3/...

Good question. We tend to document things in the code where possible.
Would there be any drawback from doing so in this case ?

>  - This is not complete to document all functionality
>    but gives an overview of the current implementation

Sounds good to me.

>  - Is there anything anyone would find helpful for me to
>    extend/write about on this?
> 
>  - It could likely get merged in with some of the work
>    that Jean-Michel is doing, and might duplicate some of
>    the parts he is documenting, but this was an aim to
>    write a single page overview as a getting started with
>    the IPU3 IPA ....

I think it's *very* useful to have an overview.

>  .../guides/ipu3-ipa-design-guide.rst          | 144 ++++++++++++++++++
>  1 file changed, 144 insertions(+)
>  create mode 100644 Documentation/guides/ipu3-ipa-design-guide.rst
> 
> diff --git a/Documentation/guides/ipu3-ipa-design-guide.rst b/Documentation/guides/ipu3-ipa-design-guide.rst
> new file mode 100644
> index 000000000000..a1d0f13fbb00
> --- /dev/null
> +++ b/Documentation/guides/ipu3-ipa-design-guide.rst
> @@ -0,0 +1,144 @@
> +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 accellerator cluster provided by the ImgU ISP.

I've commented on the expression "accelerator cluster" when reviewing
Jean-Michel's series. I've then seen that it's mentioned in the IPU3
kernel documentation in one place, I suppose that's where it comes from.
>From what I understand, the expression is used to refer to a processing
block (or sub-block) that is fully implemented in hardware, as opposed
as being implemented partly or fully in the ImgU firmware. I don't think
that's relevant from an IPA point of view, it's an internal
implementation detail of the ImgU. I'd thus use "processing block" or a
similar term instead.

> +
> +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.

s/Pipeline/pipeline/

> +
> +::
> +
> +        ┌───────────────────────────────────────────┐
> +        │      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

s/Camera Sensor/camera sensor/ ?

> +it has available, along with their defaults and ranges.

s/defaults/default values/

> +
> +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
> +before the ``start()`` operation occurs.
> +
> +The IPA will map the buffers into CPU accessible memory, associated with

s/CPU accessible/CPU-accessible/

> +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,
> +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 from either before streaming
> +commenced during ``configure()`` or updated parameters when processing
> +controls.

I'm not sure about the last part, I thought the session configuration
was meant to be constant after 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 parts of 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 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.

This reminds me that we should turn the operations (actions and events)
into separate functions in the IPAIPU3Interface. Out of scope for this
patch of course.

> +
> +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.

I expect interesting research to happen in this area :-)

-- 
Regards,

Laurent Pinchart