[PATCH 1/8] libcamera: camera: Introduce metadataAvailable signal

[Cheng-Hao Yang]

Hi Jacopo,

Thank you for the patch.

On Fri, Dec 6, 2024 at 5:07 PM Jacopo Mondi
<jacopo.mondi at ideasonboard.com> wrote:
>
> Add a new signal to the Camera class that allows applications to
> receive notifications for early completion of metadata results.
>
> To avoid expensive copies of the metadata results the signal
> transports the ids of the metadata keys that are ready. Applications
> can use these ids to access the metadata results from
> Request::metadata().

Yes, it avoids expensive copies, while I think we should restrict
usages of `Request->metadata()` both in the application and pipeline
handlers as well:
1. They should both access the metadata ControlList in the
CameraManager's thread, which is the pipeline handlers' owner thread,
to avoid race conditions. (I think `std::unordered_map` of
`ControlList::controls_` is not thread-safe, as a set of the map might
lead to rehashing. Please correct me if I am wrong.)
2. Pipeline handlers should not directly modify `Request->metadata()`,
to avoid overwriting. Instead, they should utilize the helper
functions `metadataAvailable` in the third patch in this series.

I've seen (2.) has been implemented in the following patches, thanks.
Do you think we can ensure (1.) though? Maybe at least documenting it?

BR,
Harvey

>
> The signal is an opt-in feature for applications and the sum of
> all metadata results notified through this signal is available
> in Request::metadata() at request completion time.
>
> Signed-off-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>
> ---
>  include/libcamera/camera.h |  2 ++
>  src/libcamera/camera.cpp   | 42 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+)
>
> diff --git a/include/libcamera/camera.h b/include/libcamera/camera.h
> index 94cee7bd86bb..20d51c191ecd 100644
> --- a/include/libcamera/camera.h
> +++ b/include/libcamera/camera.h
> @@ -13,6 +13,7 @@
>  #include <set>
>  #include <stdint.h>
>  #include <string>
> +#include <unordered_set>
>
>  #include <libcamera/base/class.h>
>  #include <libcamera/base/flags.h>
> @@ -122,6 +123,7 @@ public:
>
>         const std::string &id() const;
>
> +       Signal<Request *, std::unordered_set<const ControlId *>> metadataAvailable;
>         Signal<Request *, FrameBuffer *> bufferCompleted;
>         Signal<Request *> requestCompleted;
>         Signal<> disconnected;
> diff --git a/src/libcamera/camera.cpp b/src/libcamera/camera.cpp
> index 4c865a46af53..63e78b24e271 100644
> --- a/src/libcamera/camera.cpp
> +++ b/src/libcamera/camera.cpp
> @@ -886,6 +886,48 @@ const std::string &Camera::id() const
>         return _d()->id_;
>  }
>
> +/**
> + * \var Camera::metadataAvailable
> + * \brief Signal emitted when metadata for a request are available
> + *
> + * The metadataAvailable signal notifies applications about the availability
> + * of metadata for a request before the request completes.
> + *
> + * As metadata results could be large in size, the signal transports the ids
> + * of the metadata that have just been made available, but the actual control
> + * values are stored in the Camera::metadata() list.
> + *
> + * Applications can access the value of the newly available metadata results
> + * with:
> + *
> + * \code
> +
> +       void metadataAvailableHandler(Request *request,
> +                                     std::unordered_set<const ControlId *> ids)
> +       {
> +               const ControlList &metadata = request->metadata();
> +
> +               for (const auto id : ids) {
> +                       ControlValue &value = metadata.get(id->id());
> +
> +                       ....
> +               }
> +       }
> +   \endcode
> + *
> + * This signal is emitted multiple times for the same request, it is in facts
> + * emitted by the framework every time a new metadata list is made available
> + * by the Camera to the application.
> + *
> + * The sum of all metadata lists reported through this signal is equal to
> + * Request::metadata() list when the Request completes.
> + *
> + * Application can opt-in to handle this signal to receive fast notifications
> + * of metadata availability or can equally access the full metadata list
> + * at Request complete time through Request::metadata() if they have no interest
> + * in early metadata notification.
> + */
> +
>  /**
>   * \var Camera::bufferCompleted
>   * \brief Signal emitted when a buffer for a request queued to the camera has
> --
> 2.47.1
>