linux-kernel - Re: [PATCHv3 7/8] videobuf2: handle V4L2_MEMORY_FLAG_NON

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Date:   Mon, 26 Jul 2021 13:22:12 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Dafna Hirschfeld <dafna.hirschfeld@...labora.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Christoph Hellwig <hch@....de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Collabora Kernel ML <kernel@...labora.com>
Subject: Re: [PATCHv3 7/8] videobuf2: handle V4L2_MEMORY_FLAG_NON_COHERENT flag

On Mon, Jul 26, 2021 at 1:20 PM Sergey Senozhatsky
<senozhatsky@...omium.org> wrote:
>
> On (21/07/26 13:05), Sergey Senozhatsky wrote:
> > On (21/07/26 10:40), Sergey Senozhatsky wrote:
> > > On (21/07/22 19:33), Dafna Hirschfeld wrote:
> > > [..]
> > > > >   int vb2_reqbufs(struct vb2_queue *q, struct v4l2_requestbuffers *req)
> > > > >   {
> > > > >         int ret = vb2_verify_memory_type(q, req->memory, req->type);
> > > > > +       u32 flags = req->flags;
> > > > >         fill_buf_caps(q, &req->capabilities);
> > > > > -       return ret ? ret : vb2_core_reqbufs(q, req->memory, 0, &req->count);
> > > > > +       validate_memory_flags(q, req->memory, &flags);
> > > > > +       req->flags = flags;
> > > >
> > > > you can do instead
> > > >
> > > > validate_memory_flags(q, req->memory, &req->flags);
> > >
> > > ->flags are u32 for create-bufs and u8 for reqi-bufs. So `*flags = <value>`
> > > can write to ->reserved[] for req-bufs (if the value is huge enough).
> >
> > I guess ->flags can become u8 for both create-bufs and req-bufs.
> > We had ->flags in both structs as u32, but then decided to leave
> > some reserved[] space in req-bufs and switched to u8 there.
>
> Something like this
>
> ---
>
> diff --git a/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst b/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst
> index a048a9f6b7b6..cf633b5a4919 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst
> @@ -112,13 +112,13 @@ than the number requested.
>         other changes, then set ``count`` to 0, ``memory`` to
>         ``V4L2_MEMORY_MMAP`` and ``format.type`` to the buffer type.
>
> -    * - __u32
> +    * - __u8
>        - ``flags``
>        - Specifies additional buffer management attributes.
>         See :ref:`memory-flags`.
>
> -    * - __u32
> -      - ``reserved``\ [6]
> +    * - __u8
> +      - ``reserved``\ [27]
>        - A place holder for future extensions. Drivers and applications
>         must set the array to zero.
>
> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> index 6edf4508c636..2ae949ec0afa 100644
> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> @@ -694,7 +694,7 @@ static void fill_buf_caps(struct vb2_queue *q, u32 *caps)
>
>  static void validate_memory_flags(struct vb2_queue *q,
>                                   int memory,
> -                                 u32 *flags)
> +                                 u8 *flags)
>  {
>         if (!q->allow_cache_hints || memory != V4L2_MEMORY_MMAP) {
>                 /*
> @@ -711,11 +711,9 @@ static void validate_memory_flags(struct vb2_queue *q,
>  int vb2_reqbufs(struct vb2_queue *q, struct v4l2_requestbuffers *req)
>  {
>         int ret = vb2_verify_memory_type(q, req->memory, req->type);
> -       u32 flags = req->flags;
>
>         fill_buf_caps(q, &req->capabilities);
> -       validate_memory_flags(q, req->memory, &flags);
> -       req->flags = flags;
> +       validate_memory_flags(q, req->memory, &req->flags);
>         return ret ? ret : vb2_core_reqbufs(q, req->memory,
>                                             req->flags, &req->count);
>  }
> @@ -990,11 +988,9 @@ int vb2_ioctl_reqbufs(struct file *file, void *priv,
>  {
>         struct video_device *vdev = video_devdata(file);
>         int res = vb2_verify_memory_type(vdev->queue, p->memory, p->type);
> -       u32 flags = p->flags;
>
>         fill_buf_caps(vdev->queue, &p->capabilities);
> -       validate_memory_flags(vdev->queue, p->memory, &flags);
> -       p->flags = flags;
> +       validate_memory_flags(vdev->queue, p->memory, &p->flags);
>         if (res)
>                 return res;
>         if (vb2_queue_is_busy(vdev, file))
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 7973aa0465d2..ad4f7cee53f2 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -2513,8 +2513,8 @@ struct v4l2_create_buffers {
>         __u32                   memory;
>         struct v4l2_format      format;
>         __u32                   capabilities;
> -       __u32                   flags;
> -       __u32                   reserved[6];
> +       __u8                    flags;
> +       __u8                    reserved[27];
>  };
>
>  /*

I don't think we want this UABI churn anymore for a trivial
simplification in the implementation.

Best regards,
Tomasz