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    SubjectRe: [RFC PATCH v1 5/7] media: v4l2: introduce two IOCTLs for face detection
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    Hi,

    On Fri, Dec 2, 2011 at 8:33 PM, Arnd Bergmann <arnd@arndb.de> wrote:
    > On Friday 02 December 2011 17:12:56 Ming Lei wrote:
    >> +/**
    >> + * struct v4l2_fd_result - VIDIOC_G_FD_RESULT argument
    >> + * @buf_index: entry, index of v4l2_buffer for face detection
    >> + * @face_cnt:  return, how many faces detected from the @buf_index
    >> + * @fd:                return, result of faces' detection
    >> + */
    >> +struct v4l2_fd_result {
    >> +       __u32   buf_index;
    >> +       __u32   face_cnt;
    >> +       __u32   reserved[6];
    >> +       struct v4l2_fd_detection *fd;
    >> +};
    >
    >
    > This data structure is not 32/64 bit safe: running a 64 bit kernel with 32 bit
    > user space will see an incompatible layout.

    I agree that this is not 32/64 bit safe, but I understand lib32 can handle
    this correctly, otherwise many 32bit applications can't run on current
    64bit kernel
    since many kernel structures used by user space contained pointer,
    such as struct v4l2_buffer, struct v4l2_ext_controls in v4l2 ABI.

    > One way to solve this is to remove the pointer and just start the array
    > directly after the __u32 members. Alternatively, you can use a __u64
    > to pass the pointer in an integer representation.

    So I think this need not to be solved.

    >
    > A nicer interface from the data structure perspective would be to
    > get rid of the array altogether and always return exactly one face.

    I choose to return array to user space since v4l2 ioctl has provided
    this kind of support, see video_usercopy().

    thanks,
    --
    Ming Lei
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