[PATCH v1 2/2] libcamera: controls: Expose string controls as `std::string_view`
Kieran Bingham
kieran.bingham at ideasonboard.com
Thu May 1 13:45:53 CEST 2025
Quoting Barnabás Pőcze (2025-05-01 11:25:12)
> 2025. 05. 01. 12:16 keltezéssel, Kieran Bingham írta:
> > Quoting Barnabás Pőcze (2025-04-25 09:18:28)
> >> Hi
> >>
> >>
> >> 2025. 04. 23. 10:33 keltezéssel, Kieran Bingham írta:
> >>> Quoting Barnabás Pőcze (2025-04-22 13:47:51)
> >>>> When retrieving the value from a `ControlValue` usually one of two
> >>>> things happen: a small, trivially copyable object is returned by
> >>>> value; or a view into the internal buffer is provided. This is true
> >>>> for everything except strings, which are returned in `std::string`,
> >>>> incurring the overhead of string construction.
> >>>>
> >>>> To guarantee no potentially "expensive" copies, use `std::string_view`
> >>>> pointing to the internal buffer to return the value. This is similar
> >>>> to how other array-like types are returned with a `Span<>`.
> >>>>
> >>>> This is an API break, but its scope is limited to just `properties::Model`.
> >>>
> >>> This breaks in CI at the build-history stage:
> >>>
> >>> https://gitlab.freedesktop.org/camera/libcamera/-/jobs/75119329#L1034
> >>>
> >>> I wonder, are we missing a dependency declaration somewhere that should
> >>> have otherwise caused something to get rebuilt? All of the other builds
> >>> seemed to have worked on that pipeline ?
> >>
> >> Yes indeed, `controls.py` is changed, but that is not considered by
> >> meson, so it does not regenerate the cpp files because neither the
> >> template nor `gen-controls.py`, etc. has changed.
> >
> > Yikes. Probably something to bring up in the meson channels for support.
> > I have no idea on a (generic/longer term) solution I'm afraid.
> >
> >> I am not sure how it could be best addressed, there is `depend_files` where
> >> `controls.py` could be specified, but I don't think it's too convenient.
> >>
> >> Or there is the `modulefinder` thing in the standard library:
> >>
> >> >>> os.getcwd()
> >> '/libcamera/utils/codegen'
> >> >>> import modulefinder
> >> >>> m = modulefinder.ModuleFinder()
> >> >>> m.run_script("./gen-controls.py")
> >> >>> m.modules["controls"]
> >> Module('controls', '/libcamera/utils/codegen/controls.py')
> >>
> >> I am wondering if this might be used to generate a dependency file that ninja
> >> can interpret. Although this approach is definitely more complex.
> >
> > Yes, I can see that could get ugly - and we're not supposed to have to
> > parse the dependencies of every tool we use. Ok - so I guess this one is
> > different because it's our own in built tooling ...
> >
> > I think we could :
> > - Contact meson developers for support
> > - Update this patch to 'also' (trivially) modify either the template or
> > gen-controls.py
> > - Ignore the build-history failure and just keep it as is..
> >
> > I think if we want to land this series - at worst I'm ok with the last
> > option - but I'd rather keep it as a last resort to make sure we
> > maintain bisectability. And for that - I think even an arbitrary comment
> > update to the gen-controls.py which could be later removed would be fine
> > if it came to it.
>
> v2 addresses this issue by adding `depend_files` to each `custom_target()`.
> Not the best solution, but I couldn't find anything better.
Excellent, that's fine with me.
There were a few other statements/questions below here... any comment? -
I won't bother asking them on the new version ... you can keep/add my
tag to the new version.
>
>
> >
> >
> >
> >>
> >>
> >> Regards,
> >> Barnabás Pőcze
> >>
> >>>
> >>>>
> >>>> Bug: https://bugs.libcamera.org/show_bug.cgi?id=256
> >>>> Signed-off-by: Barnabás Pőcze <barnabas.pocze at ideasonboard.com>
> >>>> ---
> >>>> include/libcamera/control_ids.h.in | 1 +
> >>>> include/libcamera/controls.h | 15 ++++++++-------
> >>>> src/apps/cam/capture_script.cpp | 2 +-
> >>>> src/apps/cam/main.cpp | 2 +-
> >>>> src/apps/common/dng_writer.cpp | 2 +-
> >>>> src/apps/qcam/cam_select_dialog.cpp | 6 +++---
> >>>> src/py/libcamera/py_helpers.cpp | 4 ++--
> >>>> test/controls/control_value.cpp | 4 ++--
> >>>> utils/codegen/controls.py | 2 +-
> >>>> 9 files changed, 20 insertions(+), 18 deletions(-)
> >>>>
> >>>> diff --git a/include/libcamera/control_ids.h.in b/include/libcamera/control_ids.h.in
> >>>> index 5d0594c68..41997f79f 100644
> >>>> --- a/include/libcamera/control_ids.h.in
> >>>> +++ b/include/libcamera/control_ids.h.in
> >>>> @@ -13,6 +13,7 @@
> >>>> #include <map>
> >>>> #include <stdint.h>
> >>>> #include <string>
> >>>> +#include <string_view>
> >>>>
> >>>> #include <libcamera/controls.h>
> >>>>
> >>>> diff --git a/include/libcamera/controls.h b/include/libcamera/controls.h
> >>>> index 4bfe9615c..275f45200 100644
> >>>> --- a/include/libcamera/controls.h
> >>>> +++ b/include/libcamera/controls.h
> >>>> @@ -13,6 +13,7 @@
> >>>> #include <set>
> >>>> #include <stdint.h>
> >>>> #include <string>
> >>>> +#include <string_view>
> >>>> #include <unordered_map>
> >>>> #include <vector>
> >>>>
> >>>> @@ -96,7 +97,7 @@ struct control_type<float> {
> >>>> };
> >>>>
> >>>> template<>
> >>>> -struct control_type<std::string> {
> >>>> +struct control_type<std::string_view> {
> >>>> static constexpr ControlType value = ControlTypeString;
> >>>> static constexpr std::size_t size = 0;
> >>>> };
> >>>> @@ -138,7 +139,7 @@ public:
> >>>> #ifndef __DOXYGEN__
> >>>> template<typename T, std::enable_if_t<!details::is_span<T>::value &&
> >>>> details::control_type<T>::value &&
> >>>> - !std::is_same<std::string, std::remove_cv_t<T>>::value,
> >>>> + !std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> >>>> std::nullptr_t> = nullptr>
> >>>> ControlValue(const T &value)
> >>>> : type_(ControlTypeNone), numElements_(0)
> >>>> @@ -148,7 +149,7 @@ public:
> >>>> }
> >>>>
> >>>> template<typename T, std::enable_if_t<details::is_span<T>::value ||
> >>>> - std::is_same<std::string, std::remove_cv_t<T>>::value,
> >>>> + std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> >>>> std::nullptr_t> = nullptr>
> >>>> #else
> >>>> template<typename T>
> >>>> @@ -182,7 +183,7 @@ public:
> >>>>
> >>>> #ifndef __DOXYGEN__
> >>>> template<typename T, std::enable_if_t<!details::is_span<T>::value &&
> >>>> - !std::is_same<std::string, std::remove_cv_t<T>>::value,
> >>>> + !std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> >>>> std::nullptr_t> = nullptr>
> >>>> T get() const
> >>>> {
> >>>> @@ -193,7 +194,7 @@ public:
> >>>> }
> >>>>
> >>>> template<typename T, std::enable_if_t<details::is_span<T>::value ||
> >>>> - std::is_same<std::string, std::remove_cv_t<T>>::value,
> >>>> + std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> >>>> std::nullptr_t> = nullptr>
> >>>> #else
> >>>> template<typename T>
> >>>> @@ -210,7 +211,7 @@ public:
> >>>>
> >>>> #ifndef __DOXYGEN__
> >>>> template<typename T, std::enable_if_t<!details::is_span<T>::value &&
> >>>> - !std::is_same<std::string, std::remove_cv_t<T>>::value,
> >>>> + !std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> >>>> std::nullptr_t> = nullptr>
> >>>> void set(const T &value)
> >>>> {
> >>>> @@ -219,7 +220,7 @@ public:
> >>>> }
> >>>>
> >>>> template<typename T, std::enable_if_t<details::is_span<T>::value ||
> >>>> - std::is_same<std::string, std::remove_cv_t<T>>::value,
> >>>> + std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> >>>> std::nullptr_t> = nullptr>
> >>>> #else
> >>>> template<typename T>
> >>>> diff --git a/src/apps/cam/capture_script.cpp b/src/apps/cam/capture_script.cpp
> >>>> index fdf82efc0..bc4c7c3a0 100644
> >>>> --- a/src/apps/cam/capture_script.cpp
> >>>> +++ b/src/apps/cam/capture_script.cpp
> >>>> @@ -502,7 +502,7 @@ ControlValue CaptureScript::parseScalarControl(const ControlId *id,
> >>>> break;
> >>>> }
> >>>> case ControlTypeString: {
> >>>> - value.set<std::string>(repr);
> >>>> + value.set<std::string_view>(repr);
> >>>> break;
> >>>> }
> >>>> default:
> >>>> diff --git a/src/apps/cam/main.cpp b/src/apps/cam/main.cpp
> >>>> index fa266eca6..157d238d7 100644
> >>>> --- a/src/apps/cam/main.cpp
> >>>> +++ b/src/apps/cam/main.cpp
> >>>> @@ -337,7 +337,7 @@ std::string CamApp::cameraName(const Camera *camera)
> >>>> */
> >>>> const auto &model = props.get(properties::Model);
> >>>> if (model)
> >>>> - name = "'" + *model + "' ";
> >>>> + name.append("'").append(*model).append("' ");
> >
> > Aha, ok so this is the API breakage ... So we'll lose the ability to
> > flexibly represent strings ?
> >
> > Does this append have to construct a string ? (i.e. are we going to end
> > up just 'moving' the new construction?
> >
> > Any help/benefit from using include/libcamera/base/utils.h
> > libcamera::join();? Ah - no that would only add separateors - what we
> > would need as a string 'wrap' to allow wrapping the model with '
> > characters.
> >
> > There are other occasions where I've thought about adding a wrap ..
> > wrapper function but it's probably overkill.
> >
> >
> >
> >>>> }
> >>>>
> >>>> name += "(" + camera->id() + ")";
> >>>> diff --git a/src/apps/common/dng_writer.cpp b/src/apps/common/dng_writer.cpp
> >>>> index ac4619511..8d57023e1 100644
> >>>> --- a/src/apps/common/dng_writer.cpp
> >>>> +++ b/src/apps/common/dng_writer.cpp
> >>>> @@ -564,7 +564,7 @@ int DNGWriter::write(const char *filename, const Camera *camera,
> >>>>
> >>>> const auto &model = cameraProperties.get(properties::Model);
> >>>> if (model) {
> >>>> - TIFFSetField(tif, TIFFTAG_MODEL, model->c_str());
> >>>> + TIFFSetField(tif, TIFFTAG_MODEL, std::string(*model).c_str());
> >>>> /* \todo set TIFFTAG_UNIQUECAMERAMODEL. */
> >
> > This looks like a new construction ... does this make a copy ?
> >
> >
> > As far as I can tell - this is functionally correct/valid - and all the
> > build tests (except the build-history) work - so I'd put :
> >
> >
> > Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> >
> > Curious to see if this is something that provides enough 'benefit' over
> > changing how string controls are handled vs potential copies.
> >
> > Do we ever expect strings to be expensive?
> >
> > I could actually imagine some use cases where we could start making
> > internal/debug controls that might report larger strings in fact ... so
> > it might actaully be helpful there!
> >
> >
> >
> >>>> }
> >>>>
> >>>> diff --git a/src/apps/qcam/cam_select_dialog.cpp b/src/apps/qcam/cam_select_dialog.cpp
> >>>> index 6b6d0713c..9f25ba091 100644
> >>>> --- a/src/apps/qcam/cam_select_dialog.cpp
> >>>> +++ b/src/apps/qcam/cam_select_dialog.cpp
> >>>> @@ -114,8 +114,8 @@ void CameraSelectorDialog::updateCameraInfo(QString cameraId)
> >>>> cameraLocation_->setText("Unknown");
> >>>> }
> >>>>
> >>>> - const auto &model = properties.get(libcamera::properties::Model)
> >>>> - .value_or("Unknown");
> >>>> + const auto model = properties.get(libcamera::properties::Model)
> >>>> + .value_or("Unknown");
> >>>>
> >>>> - cameraModel_->setText(QString::fromStdString(model));
> >>>> + cameraModel_->setText(QString::fromUtf8(model.data(), model.length()));
> >>>> }
> >>>> diff --git a/src/py/libcamera/py_helpers.cpp b/src/py/libcamera/py_helpers.cpp
> >>>> index 1ad1d4c1a..8c55ef845 100644
> >>>> --- a/src/py/libcamera/py_helpers.cpp
> >>>> +++ b/src/py/libcamera/py_helpers.cpp
> >>>> @@ -47,7 +47,7 @@ py::object controlValueToPy(const ControlValue &cv)
> >>>> case ControlTypeFloat:
> >>>> return valueOrTuple<float>(cv);
> >>>> case ControlTypeString:
> >>>> - return py::cast(cv.get<std::string>());
> >>>> + return py::cast(cv.get<std::string_view>());
> >>>> case ControlTypeSize: {
> >>>> const Size *v = reinterpret_cast<const Size *>(cv.data().data());
> >>>> return py::cast(v);
> >>>> @@ -88,7 +88,7 @@ ControlValue pyToControlValue(const py::object &ob, ControlType type)
> >>>> case ControlTypeFloat:
> >>>> return controlValueMaybeArray<float>(ob);
> >>>> case ControlTypeString:
> >>>> - return ControlValue(ob.cast<std::string>());
> >>>> + return ControlValue(ob.cast<std::string_view>());
> >>>> case ControlTypeRectangle:
> >>>> return controlValueMaybeArray<Rectangle>(ob);
> >>>> case ControlTypeSize:
> >>>> diff --git a/test/controls/control_value.cpp b/test/controls/control_value.cpp
> >>>> index 5084fd0cf..032050a77 100644
> >>>> --- a/test/controls/control_value.cpp
> >>>> +++ b/test/controls/control_value.cpp
> >>>> @@ -318,7 +318,7 @@ protected:
> >>>> /*
> >>>> * String type.
> >>>> */
> >>>> - std::string string{ "libcamera" };
> >>>> + std::string_view string{ "libcamera" };
> >>>> value.set(string);
> >>>> if (value.isNone() || !value.isArray() ||
> >>>> value.type() != ControlTypeString ||
> >>>> @@ -327,7 +327,7 @@ protected:
> >>>> return TestFail;
> >>>> }
> >>>>
> >>>> - if (value.get<std::string>() != string) {
> >>>> + if (value.get<std::string_view>() != string) {
> >>>> cerr << "Control value mismatch after setting to string" << endl;
> >>>> return TestFail;
> >>>> }
> >>>> diff --git a/utils/codegen/controls.py b/utils/codegen/controls.py
> >>>> index e51610481..9399727bd 100644
> >>>> --- a/utils/codegen/controls.py
> >>>> +++ b/utils/codegen/controls.py
> >>>> @@ -111,7 +111,7 @@ class Control(object):
> >>>> size = self.__data.get('size')
> >>>>
> >>>> if typ == 'string':
> >>>> - return 'std::string'
> >>>> + return 'std::string_view'
> >>>>
> >>>> if self.__size is None:
> >>>> return typ
> >>>> --
> >>>> 2.49.0
> >>>>
> >> @
>
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