libcamera: ipa_manager: Proxy open-source IPAs to a thread

While closed-source IPA modules will always be sandboxed, open-source
IPA modules may be run in the main libcamera process or be sandboxed,
depending on platform configuration. These two models exhibit very
different timings, which require extensive testing with both
configurations.

When run into the main libcamera process, IPA modules are executed in
the pipeline handler thread (which is currently a global CameraManager
thread). Time-consuming operations in the IPA may thus slow down the
pipeline handler and compromise real-time behaviour. At least some
pipeline handlers will thus likely spawn a thread to isolate the IPA,
leading to code duplication in pipeline handlers.

Solve both issues by always proxying IPA modules. For open-source IPA
modules that run in the libcamera process, a new IPAProxyThread class is
added to run the IPA in a separate thread.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[Niklas: Move thread start/stop of thread into start()/stop()]
Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/ipa_interface.cpp

@@ -528,6 +528,9 @@ namespace libcamera {
  * \a data.operation field, as defined by the IPA protocol, and the rest of the
  * \a data is interpreted accordingly. The pipeline handler shall queue the
  * action and execute it as appropriate.
+ *
+ * The signal is only emitted when the IPA is running, that is after start() and
+ * before stop() have been called.
  */
 
 } /* namespace libcamera */