[libcamera-devel] [PATCH v5 1/4] libcamera: utils: Add helper class for std::chrono::duration
Naushir Patuck
naush at raspberrypi.com
Tue Jun 8 11:33:42 CEST 2021
Hi Laurent,
Thank you for your feedback.
On Tue, 8 Jun 2021 at 02:09, Laurent Pinchart <
laurent.pinchart at ideasonboard.com> wrote:
> Hi Naush,
>
> Thank you for the patch.
>
> 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
> > utils::Duration value in stream objects. Currently, this will display
> > 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.
>
>
> 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!
> +#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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