[libcamera-devel] [PATCH v5 1/4] libcamera: utils: Add helper class for std::chrono::duration
Laurent Pinchart
laurent.pinchart at ideasonboard.com
Tue Jun 8 11:42:38 CEST 2021
Hi Naush,
On Tue, Jun 08, 2021 at 10:33:42AM +0100, Naushir Patuck wrote:
> On Tue, 8 Jun 2021 at 02:09, Laurent Pinchart wrote:
> > On Tue, May 25, 2021 at 12:42:38PM +0100, Naushir Patuck wrote:
> > > A new utils::Duration class is defined to represent a
> > > std::chrono::duration type with double precision nanosecond timebase.
> > > Using a double minimises the loss of precision when converting timebases.
> > > This helper class may be used by IPAs to represent variables such as
> > frame
> > > durations and exposure times.
> >
> > Do we really need double precision ? Have you noticed loss of precision
> > in calculations that would be caused by usage of a 64-bit integer ? I'm
> > fine keeping the double for now, but moving forward towards more
> > widespread usage of std::chrono in libcamera, and especially in its
> > public API, I'd prefer using an integer if possible.
>
> I've used double precision as a catch all more than anything else here.
> I don't expect a precision problem with replacing it with int64 in the
> future.
>
> > > An operator << overload is define to help with displaying
> >
> > s/define/defined/
>
> Ack
>
> > > the duration value in microseconds.
> > >
> > > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
> > > Reviewed-by: David Plowman <david.plowman at raspberrypi.com>
> > > Reviewed-by: Jacopo Mondi <jacopo at jmondi.org>
> > > ---
> > > include/libcamera/internal/utils.h | 42 ++++++++++++++++++++++++++++++
> > > src/libcamera/utils.cpp | 37 ++++++++++++++++++++++++++
> > > 2 files changed, 79 insertions(+)
> > >
> > > diff --git a/include/libcamera/internal/utils.h b/include/libcamera/internal/utils.h
> > > index 83dada7cc16c..f536f2267d70 100644
> > > --- a/include/libcamera/internal/utils.h
> > > +++ b/include/libcamera/internal/utils.h
> > > @@ -316,8 +316,50 @@ auto enumerate(T (&iterable)[N]) -> details::enumerate_adapter<T *>
> > > }
> > > #endif
> > >
> > > +class Duration : public std::chrono::duration<double, std::nano>
> > > +{
> > > + using BaseDuration = std::chrono::duration<double, std::nano>;
> > > +
> > > +public:
> > > + Duration() = default;
> > > +
> > > + template<typename Rep, typename Period>
> > > + constexpr Duration(const std::chrono::duration<Rep, Period> &d)
> > > + : BaseDuration(d)
> > > + {
> > > + }
> > > +
> > > + template<typename Period>
> > > + double get() const
> > > + {
> > > + auto const c = std::chrono::duration_cast<std::chrono::duration<double, Period>>(*this);
> > > + return c.count();
> > > + }
> > > +
> > > + explicit constexpr operator bool() const
> > > + {
> > > + return *this != BaseDuration::zero();
> > > + }
> > > +};
> > > +
> > > } /* namespace utils */
> > >
> > > +#ifndef __DOXYGEN__
> > > +template<class CharT, class Traits>
> > > +static inline std::basic_ostream<CharT, Traits> &operator<<(std::basic_ostream<CharT, Traits> &os,
> > > + const utils::Duration &d)
> > > +{
> > > + std::basic_ostringstream<CharT, Traits> s;
> > > +
> > > + s.flags(os.flags());
> > > + s.imbue(os.getloc());
> > > + s.setf(std::ios_base::fixed, std::ios_base::floatfield);
> > > + s.precision(2);
> > > + s << d.get<std::micro>() << "us";
> > > + return os << s.str();
> > > +}
> >
> > It would be nice to avoid inlining this. What do you think of the
> > following ?
> >
> > diff --git a/include/libcamera/internal/utils.h
> > b/include/libcamera/internal/utils.h
> > index f536f2267d70..15beb0f44172 100644
> > --- a/include/libcamera/internal/utils.h
> > +++ b/include/libcamera/internal/utils.h
> > @@ -346,18 +346,8 @@ public:
> >
> > #ifndef __DOXYGEN__
> > template<class CharT, class Traits>
> > -static inline std::basic_ostream<CharT, Traits>
> > &operator<<(std::basic_ostream<CharT, Traits> &os,
> > - const
> > utils::Duration &d)
> > -{
> > - std::basic_ostringstream<CharT, Traits> s;
> > -
> > - s.flags(os.flags());
> > - s.imbue(os.getloc());
> > - s.setf(std::ios_base::fixed, std::ios_base::floatfield);
> > - s.precision(2);
> > - s << d.get<std::micro>() << "us";
> > - return os << s.str();
> > -}
> > +std::basic_ostream<CharT, Traits> &operator<<(std::basic_ostream<CharT,
> > Traits> &os,
> > + const utils::Duration &d);
> > #endif
> >
> > } /* namespace libcamera */
> > diff --git a/src/libcamera/utils.cpp b/src/libcamera/utils.cpp
> > index 6624ff74cdc7..cdfc6e104b52 100644
> > --- a/src/libcamera/utils.cpp
> > +++ b/src/libcamera/utils.cpp
> > @@ -545,4 +545,26 @@ std::string libcameraSourcePath()
> >
> > } /* namespace utils */
> >
> > +#ifndef __DOXYGEN__
> > +template<class CharT, class Traits>
> > +std::basic_ostream<CharT, Traits> &operator<<(std::basic_ostream<CharT,
> > Traits> &os,
> > + const utils::Duration &d)
> > +{
> > + std::basic_ostringstream<CharT, Traits> s;
> > +
> > + s.flags(os.flags());
> > + s.imbue(os.getloc());
> > + s.setf(std::ios_base::fixed, std::ios_base::floatfield);
> > + s.precision(2);
> > + s << d.get<std::micro>() << "us";
> > + return os << s.str();
> > +}
> > +
> > +template
> > +std::basic_ostream<char, std::char_traits<char>> &
> > +operator<< <char, std::char_traits<char>>(
> > + std::basic_ostream<char, std::char_traits<char>> &os,
> > + const utils::Duration &d);
> > +#endif /* __DOXYGEN__ */
> > +
> > } /* namespace libcamera */
> >
>
> Yes, I can replace the inline function with the above implementation in the
> next revision.
Thanks. I'll then merge it right away.
> > On a side node, it took me quite some time to debug linker errors. Turns
> > out that
> >
> > template<>
> > std::basic_ostream<char, std::char_traits<char>> &
> > operator<< <char, std::char_traits<char>>(
> > std::basic_ostream<char, std::char_traits<char>> &os,
> > const utils::Duration &d);
> >
> > in utils.cpp doesn't instantiate the template specialization, while
> >
> > template
> > std::basic_ostream<char, std::char_traits<char>> &
> > operator<< <char, std::char_traits<char>>(
> > std::basic_ostream<char, std::char_traits<char>> &os,
> > const utils::Duration &d);
> >
> > does.
>
> Interestingly, both the above forms work on my builds with gcc 9.3.0.
> I didn't even know that "template" without arrow brackets was valid!
Neither did I, and I've refrained from researching this :-) I was
testing with gcc 11 and clang 11.
> > +#endif
>
> > > +
> > > } /* namespace libcamera */
> > >
> > > #endif /* __LIBCAMERA_INTERNAL_UTILS_H__ */
> > > diff --git a/src/libcamera/utils.cpp b/src/libcamera/utils.cpp
> > > index 826144d3c837..6624ff74cdc7 100644
> > > --- a/src/libcamera/utils.cpp
> > > +++ b/src/libcamera/utils.cpp
> > > @@ -506,6 +506,43 @@ std::string libcameraSourcePath()
> > > * loop, iterates over an indexed view of the \a iterable
> > > */
> > >
> > > +/**
> > > + * \class Duration
> > > + * \brief Helper class from std::chrono::duration that represents a time
> > > + * duration in nanoseconds with double precision
> > > + */
> > > +
> > > +/**
> > > + * \fn Duration::Duration(const std::chrono::duration<Rep, Period> &d)
> > > + * \brief Copy constructor from a \a std::chrono::duration type
> >
> > It's not a copy constructor, as d can be of a different type than
> > Duration. How about
> >
> > * \brief Construct a Duration by converting an arbitrary
> > std::chrono::duration
>
> Ack
>
> > > + * \param[in] d The std::chrono::duration object to convert from
> > > + *
> > > + * Constructs a \a Duration object from a \a std::chrono::duration object,
> > > + * internally represented in double precision with nanoseconds ticks
> >
> > This should probably updated accordingly, I find the the second part a
> > bit misleading, it seems to imply that d is of double precision with
> > nanoseconds ticks.
>
> Ack
>
> > Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> >
> > > + */
> > > +
> > > +/**
> > > + * \fn Duration::get<Period>()
> > > + * \brief Retrieve the tick count, converted to the timebase provided by the
> > > + * template argument Period of type \a std::ratio
> > > + *
> > > + * A typical usage example is given below:
> > > + *
> > > + * \code{.cpp}
> > > + * utils::Duration d = 5s;
> > > + * double d_in_ms = d.get<std::milli>();
> > > + * \endcode
> > > + *
> > > + * \return The tick count of the Duration expressed in \a Period
> > > + */
> > > +
> > > +/**
> > > + * \fn Duration::operator bool()
> > > + * \brief Boolean operator to test if a \a Duration holds a non-zero time value
> > > + *
> > > + * \return True if \a Duration is a non-zero time value, False otherwise
> > > + */
> > > +
> > > } /* namespace utils */
> > >
> > > } /* namespace libcamera */
--
Regards,
Laurent Pinchart
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