[PATCH/RFC 21/32] libcamera: camera_sensor: Sort factories by priority

[Laurent Pinchart]

Hi Stefan,

On Wed, Mar 06, 2024 at 06:08:53PM +0100, Stefan Klug wrote:
> On Fri, Mar 01, 2024 at 11:21:10PM +0200, Laurent Pinchart wrote:
> > In order to support a default implementation for camera sensors when no
> > better implementation matches, libcamera needs to try "specialized"
> > implementations first and pick the default last. Make this possible by
> > adding a priority value for factories. Newly registered factories are
> > inserted in the factories list sorted by descending priority, and the
> > default factory uses a negative priority to be inserted as the last
> > element.
> > 
> > This mechanism may be a bit overkill in the sense that there is no
> > expected use cases for priorities other than trying the default last,
> > but the implementation is simple and easy to understand.
> > 
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  include/libcamera/internal/camera_sensor.h    | 18 +++++++---
> >  src/libcamera/sensor/camera_sensor.cpp        | 34 +++++++++++++++++--
> >  src/libcamera/sensor/camera_sensor_legacy.cpp |  2 +-
> >  3 files changed, 45 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/libcamera/internal/camera_sensor.h b/include/libcamera/internal/camera_sensor.h
> > index d37e66773195..59785ff62019 100644
> > --- a/include/libcamera/internal/camera_sensor.h
> > +++ b/include/libcamera/internal/camera_sensor.h
> > @@ -79,11 +79,13 @@ public:
> >  class CameraSensorFactoryBase
> >  {
> >  public:
> > -	CameraSensorFactoryBase();
> > +	CameraSensorFactoryBase(int priority);
> >  	virtual ~CameraSensorFactoryBase() = default;
> >  
> >  	static std::unique_ptr<CameraSensor> create(MediaEntity *entity);
> >  
> > +	int priority() const { return priority_; }
> > +
> >  private:
> >  	LIBCAMERA_DISABLE_COPY_AND_MOVE(CameraSensorFactoryBase)
> >  
> > @@ -93,14 +95,16 @@ private:
> >  
> >  	virtual std::variant<std::unique_ptr<CameraSensor>, int>
> >  	match(MediaEntity *entity) const = 0;
> > +
> > +	int priority_;
> >  };
> >  
> >  template<typename _CameraSensor>
> >  class CameraSensorFactory final : public CameraSensorFactoryBase
> >  {
> >  public:
> > -	CameraSensorFactory()
> > -		: CameraSensorFactoryBase()
> > +	CameraSensorFactory(int priority = 0)
> 
> Why the default value? Wouldn't it be clearer to require a priority.
> This would prevent and accidential registration without thinking about
> that value and running into unexpected runtime issue.

Given that I expect few CameraSensor implementations to be added to
libcamera, I'm not sure we'll ever have such accidents, but I can
require the priority.

> > +		: CameraSensorFactoryBase(priority)
> >  	{
> >  	}
> >  
> > @@ -112,7 +116,11 @@ private:
> >  	}
> >  };
> >  
> > -#define REGISTER_CAMERA_SENSOR(sensor) \
> > -	static CameraSensorFactory<sensor> global_##sensor##Factory{};
> > +#ifndef __DOXYGEN__
> > +#define REGISTER_CAMERA_SENSOR(sensor, ...) \
> > +	static CameraSensorFactory<sensor> global_##sensor##Factory{ __VA_ARGS__ };
> > +#else
> > +#define REGISTER_CAMERA_SENSOR(sensor, priority)
> > +#endif
> >  
> >  } /* namespace libcamera */
> > diff --git a/src/libcamera/sensor/camera_sensor.cpp b/src/libcamera/sensor/camera_sensor.cpp
> > index 7abbe2c76596..e7b526c2f050 100644
> > --- a/src/libcamera/sensor/camera_sensor.cpp
> > +++ b/src/libcamera/sensor/camera_sensor.cpp
> > @@ -348,11 +348,13 @@ CameraSensor::~CameraSensor() = default;
> >  
> >  /**
> >   * \brief Construct a camera sensor factory base
> > + * \param[in] priority Priority order for factory selection
> >   *
> >   * Creating an instance of the factory base registers it with the global list of
> >   * factories, accessible through the factories() function.
> >   */
> > -CameraSensorFactoryBase::CameraSensorFactoryBase()
> > +CameraSensorFactoryBase::CameraSensorFactoryBase(int priority)
> > +	: priority_(priority)
> >  {
> >  	registerFactory(this);
> >  }
> > @@ -361,6 +363,12 @@ CameraSensorFactoryBase::CameraSensorFactoryBase()
> >   * \brief Create an instance of the CameraSensor corresponding to a media entity
> >   * \param[in] entity The media entity on the source end of the sensor
> >   *
> > + * When multiple factories match the same \a entity, this function selects the
> > + * matching factory with the highest priority as specified to the
> > + * REGISTER_CAMERA_SENSOR() macro at factory registration time. If multiple
> > + * matching factories have the same highest priority value, which factory gets
> > + * selected is undefined and may vary between runs.
> 
> That doesn't feel good. Shouldn't we error out if multiple factories
> have the same priority? As far as I understand the system all
> factories are registered inside libcamera, so all priorities are under
> our control. Maybe a bit academical but who knows how many factories
> the future brings :-)

We could, but we would then have to try and match all the factories of
the same priority. As I expect all factories but the legacy to have
priority 0, it means libcamera will try to match all the non-legacy
factories, slowing down the initialization process.

> > + *
> >   * \return A unique pointer to a new instance of the CameraSensor subclass
> >   * matching the entity, or a null pointer if no such factory exists
> >   */
> > @@ -387,8 +395,17 @@ std::unique_ptr<CameraSensor> CameraSensorFactoryBase::create(MediaEntity *entit
> >  	return nullptr;
> >  }
> >  
> > +/**
> > + * \fn CameraSensorFactoryBase::priority()
> > + * \brief Retrieve the priority value for the factory
> > + * \return The priority value for the factory
> > + */
> > +
> >  /**
> >   * \brief Retrieve the list of all camera sensor factories
> > + *
> > + * The factories are sorted in decreasing priority order.
> > + *
> >   * \return The list of camera sensor factories
> >   */
> >  std::vector<CameraSensorFactoryBase *> &CameraSensorFactoryBase::factories()
> > @@ -411,7 +428,12 @@ void CameraSensorFactoryBase::registerFactory(CameraSensorFactoryBase *factory)
> >  	std::vector<CameraSensorFactoryBase *> &factories =
> >  		CameraSensorFactoryBase::factories();
> >  
> > -	factories.push_back(factory);
> > +	auto pos = std::upper_bound(factories.begin(), factories.end(), factory,
> > +				    [](const CameraSensorFactoryBase *value,
> > +				       const CameraSensorFactoryBase *elem) {
> > +					    return value->priority() > elem->priority();
> > +				    });
> > +	factories.insert(pos, factory);
> >  }
> >  
> >  /**
> > @@ -437,9 +459,10 @@ void CameraSensorFactoryBase::registerFactory(CameraSensorFactoryBase *factory)
> >   */
> >  
> >  /**
> > - * \def REGISTER_CAMERA_SENSOR(sensor)
> > + * \def REGISTER_CAMERA_SENSOR(sensor, priority)
> >   * \brief Register a camera sensor type to the sensor factory
> >   * \param[in] sensor Class name of the CameraSensor derived class to register
> > + * \param[in] priority Priority order for factory selection
> >   *
> >   * Register a CameraSensor subclass with the factory and make it available to
> >   * try and match sensors. The subclass needs to implement a static match
> > @@ -457,6 +480,11 @@ void CameraSensorFactoryBase::registerFactory(CameraSensorFactoryBase *factory)
> >   *   creation succeeded ;
> >   * - A non-zero error code if the entity matched and the creation failed ; or
> >   * - A zero error code if the entity didn't match.
> > + *
> > + * When multiple factories can support the same MediaEntity (as in the match()
> > + * function of multiple factories returning true for the same entity), the \a
> > + * priority argument selects which factory will be used. See
> > + * CameraSensorFactoryBase::create() for more information.
> >   */
> >  
> >  } /* namespace libcamera */
> > diff --git a/src/libcamera/sensor/camera_sensor_legacy.cpp b/src/libcamera/sensor/camera_sensor_legacy.cpp
> > index 34677339241c..f9f889a125d0 100644
> > --- a/src/libcamera/sensor/camera_sensor_legacy.cpp
> > +++ b/src/libcamera/sensor/camera_sensor_legacy.cpp
> > @@ -1010,6 +1010,6 @@ std::string CameraSensorLegacy::logPrefix() const
> >  	return "'" + entity_->name() + "'";
> >  }
> >  
> > -REGISTER_CAMERA_SENSOR(CameraSensorLegacy)
> > +REGISTER_CAMERA_SENSOR(CameraSensorLegacy, -100)
> >  
> >  } /* namespace libcamera */

-- 
Regards,

Laurent Pinchart