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    SubjectRe: [RFC PATCH v1 5/7] media: v4l2: introduce two IOCTLs for face detection
    Date
    On Tuesday 06 December 2011, Ming Lei wrote:
    > > Using an array added to the end of the v4l2_fd_result structure
    > > rather than a pointer would really make this easier IMHO.
    >
    > I have tried to do this, but video_usercopy needs a few changes
    > to handle array args if no indirect pointer is passed to kernel.

    Ah, I see. Or you would have to encode the array size into the
    ioctl command, which is also ugly in a different way.

    > I am not sure if media guys are happy to accept the changes, :-)

    Maybe Mauro can comment on which solution he prefers then, given
    the choice between:

    1. adding another handler in drivers/media/video/v4l2-compat-ioctl32.c

    2. passing a pointer that is casted to __u64 in user space an back
    in the kernel

    3. extending video_usercopy in some way to make this work, preferably
    in a generic way.

    4. using a variable command number like
    #define VIDIOC_G_FD_RESULT(num) _IOC(_IOC_READ|_IOC_WRITE,'V', 95, \
    sizeof(struct v4l2_fd_result) + (num) * sizeof(struct v4l2_fd_detection)

    5. requiring the interface to be simplified to return only a single
    struct v4l2_fd_detection at a time

    I agree that none of these are nice. My preferred option would be last one,
    but I don't know how performance critical the interface is or if it would
    cause any races that you want to avoid.

    Arnd


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