[libcamera-devel] [PATCH v5 2/4] libcamera: V4L2Device: Use std::vector for v4l2_ext_control in getControls()

[Hirokazu Honda]

Hi Laurent,

On Mon, Apr 26, 2021 at 9:12 AM Laurent Pinchart
<laurent.pinchart at ideasonboard.com> wrote:
>
> Hi Hiro,
>
> Thank you for the patch.
>
> On Fri, Apr 23, 2021 at 06:36:51PM +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 | 41 +++++++++++++++++------------------
> >  1 file changed, 20 insertions(+), 21 deletions(-)
> >
> > diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
> > index ce2860c4..bbe8f154 100644
> > --- a/src/libcamera/v4l2_device.cpp
> > +++ b/src/libcamera/v4l2_device.cpp
> > @@ -173,13 +173,18 @@ void V4L2Device::close()
> >   */
> >  ControlList V4L2Device::getControls(const std::vector<uint32_t> &ids)
> >  {
> > -     unsigned int count = ids.size();
> > -     if (count == 0)
> > +     if (ids.empty())
> >               return {};
> >
> >       ControlList ctrls{ controls_ };
> > +     std::vector<v4l2_ext_control> v4l2Ctrls(ids.size());
> > +     memset(v4l2Ctrls.data(), 0, sizeof(v4l2_ext_control) * ctrls.size());
> > +
> > +     for (size_t i = 0, j = 0; j < ids.size(); ++j) {
> > +             const unsigned int id = ids[j];
> > +             if (ctrls.contains(id))
> > +                     continue;
> >
> > -     for (uint32_t id : ids) {
> >               const auto iter = controls_.find(id);
> >               if (iter == controls_.end()) {
> >                       LOG(V4L2, Error)
> > @@ -187,20 +192,12 @@ ControlList V4L2Device::getControls(const std::vector<uint32_t> &ids)
> >                       return {};
> >               }
> >
> > -             ctrls.set(id, {});
> > -     }
> > -
> > -     struct v4l2_ext_control v4l2Ctrls[count];
> > -     memset(v4l2Ctrls, 0, sizeof(v4l2Ctrls));
> > -
> > -     unsigned int i = 0;
> > -     for (auto &ctrl : ctrls) {
> > -             unsigned int id = ctrl.first;
> > +             v4l2_ext_control &v4l2Ctrl = v4l2Ctrls[i++];
>
> You increase i here, ...
>
> >               const struct v4l2_query_ext_ctrl &info = controlInfo_[id];
> > +             ControlValue value{};
>
> ControlValue has a default constructor, no need for {}.
>
> >
> >               if (info.flags & V4L2_CTRL_FLAG_HAS_PAYLOAD) {
> >                       ControlType type;
> > -
> >                       switch (info.type) {
> >                       case V4L2_CTRL_TYPE_U8:
> >                               type = ControlTypeByte;
> > @@ -213,7 +210,6 @@ 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();
> >
> > @@ -221,21 +217,23 @@ ControlList V4L2Device::getControls(const std::vector<uint32_t> &ids)
> >                       v4l2Ctrls[i].size = data.size();
>
> ... and use it here. I don't think that's correct.
>
> >               }
> >
> > -             v4l2Ctrls[i].id = id;
> > -             i++;
> > +             v4l2Ctrl.id = id;
>
> Should we move this just after the declaration of v4l2Ctrl above ?
>
> > +             ctrls.set(id, std::move(value));
>
> v4l2Ctrls contains pointers to data stored in the ControlValue
> instances. As far as I can tell the pointers will remain valid, but
> that's very dependent on the internals of ControlList.
>
> To be honest, I'm not very fond of patches 1/4 and 2/4 in this series.
> They make the code more fragile and possibly a bit less efficient
> (although that's likely not significant, as there shouldn't be thousands
> of controls in the list). The VLA removal is fine, but the rest doesn't
> bring much value in my opinion.
>

Can you clarify more how the new code is fragile and less efficient?
You're right, I am sorry that this implementation has some problems.
But I think 1/4 is worth doing and correct, and 2/4 is good if I fix the bugs?

-Hiro
> Let's split this in two parts, with the fixes first, and the rework on
> top, so both can be discussed separately. I've posted the former as a
> v6.
>
> >       }
> >
> > +     v4l2Ctrls.resize(ctrls.size());
> > +
> >       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 {};
> > @@ -244,10 +242,11 @@ 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());
> >
> >       return ctrls;
> >  }
>
> --
> Regards,
>
> Laurent Pinchart