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

[Hirokazu Honda]

Hi Kieran,

On Wed, Apr 14, 2021 at 6:58 AM Kieran Bingham
<kieran.bingham at ideasonboard.com> wrote:
>
> Hi Hiro,
>
> On 13/04/2021 07:19, 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 | 44 +++++++++++++++++------------------
> >  1 file changed, 21 insertions(+), 23 deletions(-)
> >
> > diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
> > index decd19ef..78c289c2 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)
> >  {
> > -     unsigned int count = ids.size();
> > -     if (count == 0)
> > +     if (ids.empty())
> >               return {};
> >
> >       ControlList ctrls{ controls_ };
> > @@ -196,21 +195,26 @@ 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());
>
> The v4l2ctrls array is created of size count which was ids.size();
> Shouldn't we reserve this as ids.size() here instead of ctrls.size()?
>

Yeah, I am also confused by the point upon creating this patch.
The original implementation seems to contain a bug;
count = id.size() and v4l2Ctrls's size is count, but the iteration is
up to ctrls.size() and thus v4l2Ctrls[ctrls.size() - 1] is set.
I expect this is out-of-bounds bug.
This patch fixes it by setting v4l2Ctrls.size() to ctrls.size() and
keeping the original return semantics.
However, I still don't get the original return semantics is correct.
The doc states "This method reads the value of all controls contained
in \a ids, and returns their values as a ControlList."
The implementation looks to return all the control values regardless of ids.
+Laurent Pinchart do you have any idea?

>
> >       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));
>
> Can v4l2Ctrl initialisation be simplified to this?:
>         v4l2_ext_control v4l2Ctrl = {};
>
> >
> >               if (info.flags & V4L2_CTRL_FLAG_HAS_PAYLOAD) {
> > -                     ControlType type;
> > -
> >                       switch (info.type) {
> > -                     case V4L2_CTRL_TYPE_U8:
> > +                     case V4L2_CTRL_TYPE_U8: {
> > +                             ControlType type;
> >                               type = ControlTypeByte;
> > +                             ControlValue &value = ctrl.second;
> > +                             value.reserve(type, true, info.elems);
>
> Is this the only usage of type? Couldn't we just do
>  value.reserve(ControlTypeByte, true, info.elems);
>
> Or at the least make it
>
> ControlType = ControlTypeByte;
>
> rather than use an extra line.
>
>
> > +                             Span<uint8_t> data = value.data();
> > +                             v4l2Ctrl.p_u8 = data.data();
> > +                             v4l2Ctrl.size = data.size();
> >                               break;
> > +                     }
> >
> >                       default:
> >                               LOG(V4L2, Error)
> > @@ -218,30 +222,23 @@ ControlList V4L2Device::getControls(const std::vector<uint32_t> &ids)
> >                                       << info.type;
> >                               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();
>
> That switch statement must have been arranged like that to facilitate
> easily adding other types later.
>
> If we don't need to do that, can we fix this up to lower the indentation
> somehow? We're four levels in here in places :-S
>
>
> >               }
> >
> > -             v4l2Ctrls[i].id = id;
> > -             i++;
> > +             v4l2Ctrl.id = id;
> > +             v4l2Ctrls.push_back(std::move(v4l2Ctrl));
> >       }
> >
> >       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 +247,11 @@ 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());
>
> I was going to say can we pass v4l2Ctrls directly, but now I've seen
> your later patch ;=)
>
>
>
> >
> >       return ctrls;
> >  }
> >
>
> --
> Regards
> --
> Kieran