[libcamera-devel] [PATCH v8 1/4] cam: event_loop: Add timer events to event loop

[Laurent Pinchart]

Hi Eric,

On Tue, May 17, 2022 at 08:19:34PM +0100, Kieran Bingham via libcamera-devel wrote:
> Quoting Eric Curtin via libcamera-devel (2022-05-05 16:18:48)
> > Extend the EventLoop class to support periodic timer events. This can be
> > used to run tasks periodically, such as handling the event loop of SDL.
> > 
> > Also delete all events in the list, before we event_base_loopbreak.

Can you extend the commit message to explain why this is needed ?

> > Signed-off-by: Eric Curtin <ecurtin at redhat.com>
> > ---
> >  src/cam/event_loop.cpp | 33 +++++++++++++++++++++++++++++++++
> >  src/cam/event_loop.h   |  6 ++++++
> >  2 files changed, 39 insertions(+)
> > 
> > diff --git a/src/cam/event_loop.cpp b/src/cam/event_loop.cpp
> > index e25784c0..181c971c 100644
> > --- a/src/cam/event_loop.cpp
> > +++ b/src/cam/event_loop.cpp
> > @@ -47,6 +47,10 @@ int EventLoop::exec()
> >  void EventLoop::exit(int code)
> >  {
> >         exitCode_ = code;
> > +       for (auto const &e : events_) {
> > +               event_del(e->event_);
> > +       }
> 
> No need for { } on a single line statement.

And as events_ is a vector of unique_ptr, just

	events_.clear();

will do.

> > +
> >         event_base_loopbreak(base_);
> >  }
> >  
> > @@ -84,6 +88,35 @@ void EventLoop::addEvent(int fd, EventType type,
> >         events_.push_back(std::move(event));
> >  }
> >  
> > +void EventLoop::toTimeval(const std::chrono::milliseconds d, struct timeval &tv)
> > +{
> > +       tv.tv_sec = std::chrono::duration_cast<std::chrono::seconds>(d).count();
> 
> Do you need to subtract the seconds from here now?
> 
> > +       tv.tv_usec = std::chrono::duration_cast<std::chrono::microseconds>(d).count();
> 
> Or does this wrap accordingly?
> 
> I.e. would 1.5 seconds end up as :
> 	  tv.tv_sec = 1;
> 	  tv.tv_usec = 1500000;

Looks like an issue indeed. There's an implementation in
src/libcamera/base/utils.h that you can copy:

	uint64_t usecs = std::chrono::duration_cast<std::chrono::microseconds(d).count();
        tv.tv_sec = usecs / 1000000ULL;
        tv.tv_usec = usecs % 1000000ULL;

As this is only three lines of code, and used once in addTimerEvent(),
I'd just move it to that function and drom toTimeval().

> With that resolved, I think that's a 
> 
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> 
> > +}
> > +
> > +void EventLoop::addTimerEvent(const std::chrono::milliseconds d,

As this adds a periodic timer event, let's call the variable period (or
interval, up to you) instead of d.

> > +                             const std::function<void()> &callback)
> > +{
> > +       std::unique_ptr<Event> event = std::make_unique<Event>(callback);
> > +       event->event_ = event_new(base_, -1, EV_PERSIST, &EventLoop::Event::dispatch,
> > +                                 event.get());
> > +       if (!event->event_) {
> > +               std::cerr << "Failed to create timer event" << std::endl;
> > +               return;
> > +       }
> > +
> > +       struct timeval tv;
> > +       toTimeval(d, tv);
> 
> It's a shame that it's frowned upon to override std namespace to have
> custom constructors or having a construction to allow:
> 	struct timeval tv(d);
> 
> would be nice ... (but I don't think it's something we should do here).

That's something that the C++ standard library could have provided. We
have a helper function in base/utils.h but that's not public.

> > +
> > +       int ret = event_add(event->event_, &tv);
> > +       if (ret < 0) {
> > +               std::cerr << "Failed to add timer event" << std::endl;
> > +               return;
> > +       }
> > +
> > +       events_.push_back(std::move(event));
> > +}
> > +
> >  void EventLoop::dispatchCallback([[maybe_unused]] evutil_socket_t fd,
> >                                  [[maybe_unused]] short flags, void *param)
> >  {
> > diff --git a/src/cam/event_loop.h b/src/cam/event_loop.h
> > index a4613eb2..89215dce 100644
> > --- a/src/cam/event_loop.h
> > +++ b/src/cam/event_loop.h
> > @@ -14,6 +14,8 @@

You should include <chrono> here.

> >  
> >  #include <event2/util.h>
> >  
> > +using namespace std::chrono_literals;

I think this can be dropped, as you don't use literals in this file.

With these small changes you'll have my Rb tag.

> > +
> >  struct event_base;
> >  
> >  class EventLoop
> > @@ -37,6 +39,9 @@ public:
> >         void addEvent(int fd, EventType type,
> >                       const std::function<void()> &handler);
> >  
> > +       void addTimerEvent(const std::chrono::milliseconds d,
> > +                          const std::function<void()> &handler);
> > +
> >  private:
> >         struct Event {
> >                 Event(const std::function<void()> &callback);
> > @@ -57,6 +62,7 @@ private:
> >         std::list<std::unique_ptr<Event>> events_;
> >         std::mutex lock_;
> >  
> > +       void toTimeval(const std::chrono::milliseconds d, struct timeval &tv);
> >         static void dispatchCallback(evutil_socket_t fd, short flags,
> >                                      void *param);
> >         void dispatchCall();

-- 
Regards,

Laurent Pinchart