[libcamera-devel] [PATCH v2 1/3] libcamera: V4L2Device: Use std::vector for v4l2_ext_control in getControls()

[Laurent Pinchart]

Hi Hiro,

On Thu, Apr 22, 2021 at 11:09:06AM +0900, Hirokazu Honda wrote:
> On Tue, Apr 20, 2021 at 9:59 AM Laurent Pinchart wrote:
> > On Thu, Apr 15, 2021 at 01:48:52PM +0900, Hirokazu Honda wrote:
> > > The original code uses Variable-Length-Array, which is not
> > > officially supported in C++. This replaces the array with
> > > std::vector.
> > >
> > > Signed-off-by: Hirokazu Honda <hiroh at chromium.org>
> > > ---
> > >  src/libcamera/v4l2_device.cpp | 43 ++++++++++++++++-------------------
> > >  1 file changed, 20 insertions(+), 23 deletions(-)
> > >
> > > diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
> > > index 8fd79934..ee8c3fed 100644
> > > --- a/src/libcamera/v4l2_device.cpp
> > > +++ b/src/libcamera/v4l2_device.cpp
> > > @@ -173,8 +173,7 @@ void V4L2Device::close()
> > >   */
> > >  ControlList V4L2Device::getControls(const std::vector<uint32_t> &ids)
> >
> > Unrelated to this patch, it occurred to me that the function should take
> > a Span<const uint32_t> instead of a vector. Would you like to submit an
> > additional patch ?
> 
> I tried this today. Span has an issue in a call like the following?
> 
> ../src/libcamera/camera_sensor.cpp:804:43: error: no matching
> constructor for initialization of 'Span<const uint32_t>' (aka
> 'Span<const unsigned int>')
>         ControlList ctrls = subdev_->getControls({ V4L2_CID_PIXEL_RATE,

Indeed, the span class has no constructor that takes an
std::initializer_list. This could be solved by constructing the
initializer list explicitly in the caller (but would still require the
following patch).

diff --git a/include/libcamera/span.h b/include/libcamera/span.h
index 7a35806b1710..33e059aac102 100644
--- a/include/libcamera/span.h
+++ b/include/libcamera/span.h
@@ -67,6 +67,12 @@ constexpr T *data(T (&array)[N]) noexcept
 	return array;
 }

+template<typename T>
+constexpr const T *data(std::initializer_list<T> c) noexcept
+{
+	return c.begin();
+}
+
 template<std::size_t I, typename T>
 struct tuple_element;

Alternatively, we could add a consructor take takes an initializer list.
Let's postpone this for now.

> > >  {
> > > -     unsigned int count = ids.size();
> > > -     if (count == 0)
> > > +     if (ids.empty())
> > >               return {};
> > >
> > >       ControlList ctrls;
> > > @@ -196,22 +195,25 @@ ControlList V4L2Device::getControls(const std::vector<uint32_t> &ids)
> > >               ctrls.set(id, {});
> > >       }
> > >
> > > -     struct v4l2_ext_control v4l2Ctrls[count];
> > > -     memset(v4l2Ctrls, 0, sizeof(v4l2Ctrls));
> > > -
> > > -     unsigned int i = 0;
> > > +     std::vector<v4l2_ext_control> v4l2Ctrls;
> > > +     v4l2Ctrls.reserve(ctrls.size());
> >
> > This changes the behaviour slightly, in that if the ids vector contains
> > the same ID multiple times, you'll allocate less memory here. That's
> > actually better :-)
> >
> > >       for (auto &ctrl : ctrls) {
> > >               unsigned int id = ctrl.first;
> > >               const struct v4l2_query_ext_ctrl &info = controlInfo_[id];
> > > +             v4l2_ext_control v4l2Ctrl;
> > > +             memset(&v4l2Ctrl, 0, sizeof(v4l2Ctrl));
> > >
> > >               if (info.flags & V4L2_CTRL_FLAG_HAS_PAYLOAD) {
> > > -                     ControlType type;
> > > +                     ControlValue &value = ctrl.second;
> > >
> > >                       switch (info.type) {
> > > -                     case V4L2_CTRL_TYPE_U8:
> > > -                             type = ControlTypeByte;
> > > +                     case V4L2_CTRL_TYPE_U8: {
> > > +                             value.reserve(ControlTypeByte, true, info.elems);
> > > +                             Span<uint8_t> data = value.data();
> > > +                             v4l2Ctrl.p_u8 = data.data();
> > > +                             v4l2Ctrl.size = data.size();
> > >                               break;
> > > -
> > > +                     }
> > >                       default:
> > >                               LOG(V4L2, Error)
> > >                                       << "Unsupported payload control type "
> > > @@ -219,29 +221,23 @@ ControlList V4L2Device::getControls(const std::vector<uint32_t> &ids)
> > >                               return {};
> > >                       }
> > >
> > > -                     ControlValue &value = ctrl.second;
> > > -                     value.reserve(type, true, info.elems);
> > > -                     Span<uint8_t> data = value.data();
> > > -
> > > -                     v4l2Ctrls[i].p_u8 = data.data();
> > > -                     v4l2Ctrls[i].size = data.size();
> >
> > Is there a need to move this code to the switch ? It looks fairly
> > generic, it would be nice not to duplicate it when (if) we latter add
> > support for other control types (such as V4L2_CTRL_TYPE_U16).
> >
> > >               }
> > >
> > > -             v4l2Ctrls[i].id = id;
> > > -             i++;
> > > +             v4l2Ctrl.id = id;
> > > +             v4l2Ctrls.push_back(std::move(v4l2Ctrl));
> >
> > The move won't help much, it will copy the v4l2_ext_control instance.
> > Should we resize() instead of reserve(), and operate directly on the
> > vector entry in this loop ?
> >
> > >       }
> > >
> > >       struct v4l2_ext_controls v4l2ExtCtrls = {};
> > >       v4l2ExtCtrls.which = V4L2_CTRL_WHICH_CUR_VAL;
> > > -     v4l2ExtCtrls.controls = v4l2Ctrls;
> > > -     v4l2ExtCtrls.count = count;
> > > +     v4l2ExtCtrls.controls = v4l2Ctrls.data();
> > > +     v4l2ExtCtrls.count = v4l2Ctrls.size();
> > >
> > >       int ret = ioctl(VIDIOC_G_EXT_CTRLS, &v4l2ExtCtrls);
> > >       if (ret) {
> > >               unsigned int errorIdx = v4l2ExtCtrls.error_idx;
> > >
> > >               /* Generic validation error. */
> > > -             if (errorIdx == 0 || errorIdx >= count) {
> > > +             if (errorIdx == 0 || errorIdx >= v4l2Ctrls.size()) {
> > >                       LOG(V4L2, Error) << "Unable to read controls: "
> > >                                        << strerror(-ret);
> > >                       return {};
> > > @@ -250,10 +246,11 @@ ControlList V4L2Device::getControls(const std::vector<uint32_t> &ids)
> > >               /* A specific control failed. */
> > >               LOG(V4L2, Error) << "Unable to read control " << errorIdx
> > >                                << ": " << strerror(-ret);
> > > -             count = errorIdx - 1;
> > > +
> > > +             v4l2Ctrls.resize(errorIdx);
> > >       }
> > >
> > > -     updateControls(&ctrls, v4l2Ctrls, count);
> > > +     updateControls(&ctrls, v4l2Ctrls.data(), v4l2Ctrls.size());
> >
> > updateControls should also take a Span argument, that could be done in
> > the same patch that updates the argument to getControls().
> >
> > Overall this patch is good, the comments above are small issues.
> >
> > >
> > >       return ctrls;
> > >  }

-- 
Regards,

Laurent Pinchart