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

[Kieran Bingham]

Hi Hiro,

On 14/04/2021 02:45, Hirokazu Honda wrote:
> 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?

I think this is where it's important to keep distinct changes in
separate patches.

Try to make this patch perform only the conversion to the vector, with
like-for-like (bugz inclusive) functionality, and then it's easier to
discuss and fix the bug if it is there directly in it's own patch.

> 
>>
>>>       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

-- 
Regards
--
Kieran