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Message-ID: <CAAFQd5CSNbOsefMca-CXCSWrXy+3TqVRMZsUanJqJ7Mp4AK-2g@mail.gmail.com>
Date: Fri, 10 Nov 2023 17:58:06 +0900
From: Tomasz Figa <tfiga@...omium.org>
To: Benjamin Gaignard <benjamin.gaignard@...labora.com>,
Hans Verkuil <hverkuil@...all.nl>
Cc: mchehab@...nel.org, m.szyprowski@...sung.com, ming.qian@....com,
ezequiel@...guardiasur.com.ar, p.zabel@...gutronix.de,
gregkh@...uxfoundation.org, hverkuil-cisco@...all.nl,
nicolas.dufresne@...labora.com, linux-media@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-mediatek@...ts.infradead.org, linux-arm-msm@...r.kernel.org,
linux-rockchip@...ts.infradead.org, linux-staging@...ts.linux.dev,
kernel@...labora.com
Subject: Re: [PATCH v14 54/56] media: v4l2: Add DELETE_BUFS ioctl
On Thu, Nov 9, 2023 at 7:10 PM Benjamin Gaignard
<benjamin.gaignard@...labora.com> wrote:
>
>
> Le 09/11/2023 à 10:30, Tomasz Figa a écrit :
> > On Tue, Oct 31, 2023 at 05:31:02PM +0100, Benjamin Gaignard wrote:
> >> VIDIOC_DELETE_BUFS ioctl allows to delete buffers from a queue.
> >> The number of buffers to delete in given by count field of
> >> struct v4l2_delete_buffers and the range start at the index
> >> specified in the same structure.
> >>
> >> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
> >> ---
> >> .../userspace-api/media/v4l/user-func.rst | 1 +
> >> .../media/v4l/vidioc-delete-bufs.rst | 80 +++++++++++++++++++
> >> .../media/v4l/vidioc-reqbufs.rst | 1 +
> >> .../media/common/videobuf2/videobuf2-core.c | 30 +++++++
> >> .../media/common/videobuf2/videobuf2-v4l2.c | 20 +++++
> >> drivers/media/v4l2-core/v4l2-dev.c | 1 +
> >> drivers/media/v4l2-core/v4l2-ioctl.c | 17 ++++
> >> include/media/v4l2-ioctl.h | 4 +
> >> include/media/videobuf2-core.h | 12 +++
> >> include/media/videobuf2-v4l2.h | 13 +++
> >> include/uapi/linux/videodev2.h | 17 ++++
> >> 11 files changed, 196 insertions(+)
> >> create mode 100644 Documentation/userspace-api/media/v4l/vidioc-delete-bufs.rst
> >>
> >> diff --git a/Documentation/userspace-api/media/v4l/user-func.rst b/Documentation/userspace-api/media/v4l/user-func.rst
> >> index 15ff0bf7bbe6..3fd567695477 100644
> >> --- a/Documentation/userspace-api/media/v4l/user-func.rst
> >> +++ b/Documentation/userspace-api/media/v4l/user-func.rst
> >> @@ -17,6 +17,7 @@ Function Reference
> >> vidioc-dbg-g-chip-info
> >> vidioc-dbg-g-register
> >> vidioc-decoder-cmd
> >> + vidioc-delete-bufs
> >> vidioc-dqevent
> >> vidioc-dv-timings-cap
> >> vidioc-encoder-cmd
> >> diff --git a/Documentation/userspace-api/media/v4l/vidioc-delete-bufs.rst b/Documentation/userspace-api/media/v4l/vidioc-delete-bufs.rst
> >> new file mode 100644
> >> index 000000000000..4791df1395d6
> >> --- /dev/null
> >> +++ b/Documentation/userspace-api/media/v4l/vidioc-delete-bufs.rst
> >> @@ -0,0 +1,80 @@
> >> +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
> >> +.. c:namespace:: V4L
> >> +
> >> +.. _VIDIOC_DELETE_BUFS:
> >> +
> >> +************************
> >> +ioctl VIDIOC_DELETE_BUFS
> >> +************************
> >> +
> >> +Name
> >> +====
> >> +
> >> +VIDIOC_DELETE_BUFS - Deletes buffers from a queue
> >> +Drivers using this feature must expose the ``V4L2_BUF_CAP_SUPPORTS_DELETE_BUFS``
> >> +capability on the queue :c:func:`VIDIOC_REQBUFS` or :c:func:`VIDIOC_CREATE_BUFS`
> >> +are invoked.
> >> +
> >> +Synopsis
> >> +========
> >> +
> >> +.. c:macro:: VIDIOC_DELETE_BUFs
> >> +
> >> +``int ioctl(int fd, VIDIOC_DELETE_BUFs, struct v4l2_delete_buffers *argp)``
> >> +
> >> +Arguments
> >> +=========
> >> +
> >> +``fd``
> >> + File descriptor returned by :c:func:`open()`.
> >> +
> >> +``argp``
> >> + Pointer to struct :c:type:`v4l2_delete_buffers`.
> >> +
> >> +Description
> >> +===========
> >> +
> >> +Applications can optionally call the :ref:`VIDIOC_DELETE_BUFS` ioctl to
> >> +delete buffers from a queue.
> >> +
> >> +.. c:type:: v4l2_delete_buffers
> >> +
> >> +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
> >> +
> >> +.. flat-table:: struct v4l2_delete_buffers
> >> + :header-rows: 0
> >> + :stub-columns: 0
> >> + :widths: 1 1 2
> >> +
> >> + * - __u32
> >> + - ``index``
> >> + - The starting buffer index to delete.
> >> + * - __u32
> >> + - ``count``
> >> + - The number of buffers to be deleted with indices 'index' until 'index + count - 1'.
> >> + All buffers in this range must be valid and in DEQUEUED state.
> > I wonder if this wouldn't be an inconvenience for the userspace. Would
> > there be anything wrong with allowing ranges with holes?
> > (Agreed about the DEQUEUED part of course.)
>
> For me a range as to be valid but the matter of taste.
>
@Hans Verkuil do you have an opinion?
I could imagine that applications may want to just delete multiple
disjoint ranges in one ioctl, which wouldn't be possible if we
prohibit holes in the ranges.
> >
> >> + In error case errno is set to ``EINVAL`` error code and index returns the index of
> >> + the invalid buffer.
> >> + If count and index are set to 0 :ref:`VIDIOC_DELETE_BUFS` will return 0.
> > Hmm, do we care about index if count is zero?
>
> No we don't I will change that.
>
> >
> >> + * - __u32
> >> + - ``type``
> >> + - Type of the stream or buffers, this is the same as the struct
> >> + :c:type:`v4l2_format` ``type`` field. See
> >> + :c:type:`v4l2_buf_type` for valid values.
> >> + * - __u32
> >> + - ``reserved``\ [13]
> >> + - A place holder for future extensions. Drivers and applications
> >> + must set the array to zero.
> >> +
> >> +Return Value
> >> +============
> >> +
> >> +On success 0 is returned, on error -1 and the ``errno`` variable is set
> >> +appropriately. The generic error codes are described at the
> >> +:ref:`Generic Error Codes <gen-errors>` chapter.
> >> +
> >> +EBUSY
> >> + File I/O is in progress.
> >> +
> >> +EINVAL
> >> + The buffer ``index`` doesn't exist in the queue.
> >> diff --git a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> >> index 0b3a41a45d05..14d4a49c2945 100644
> >> --- a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> >> +++ b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> >> @@ -121,6 +121,7 @@ aborting or finishing any DMA in progress, an implicit
> >> .. _V4L2-BUF-CAP-SUPPORTS-M2M-HOLD-CAPTURE-BUF:
> >> .. _V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS:
> >> .. _V4L2-BUF-CAP-SUPPORTS-MAX-NUM-BUFFERS:
> >> +.. _V4L2-BUF-CAP-SUPPORTS-DELETE-BUFS:
> >>
> >> .. raw:: latex
> >>
> >> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> >> index 010a8bca0240..7068930a0ba6 100644
> >> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> >> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> >> @@ -1665,6 +1665,36 @@ int vb2_core_prepare_buf(struct vb2_queue *q, struct vb2_buffer *vb, void *pb)
> >> }
> >> EXPORT_SYMBOL_GPL(vb2_core_prepare_buf);
> >>
> >> +int vb2_core_delete_bufs(struct vb2_queue *q, unsigned int start, unsigned int count)
> >> +{
> >> + unsigned int i, ret = 0;
> >> +
> >> + if (start == 0 && count == 0)
> > I guess related to the earlier comment, but why do we care about start
> > being zero?
> >
> >> + return 0;
> >> +
> >> + mutex_lock(&q->mmap_lock);
> >> +
> >> + for (i = start; i < start + count && i < q->max_num_buffers; i++) {
> >> + struct vb2_buffer *vb = vb2_get_buffer(q, i);
> >> +
> >> + if (!vb) {
> >> + ret = -EINVAL;
> >> + goto unlock;
> >> + }
> >> + if (vb->state != VB2_BUF_STATE_DEQUEUED) {
> >> + ret = -EINVAL;
> >> + goto unlock;
> >> + }
> >> + }
> >> + __vb2_queue_free(q, start, count);
> >> + dprintk(q, 2, "buffers deleted\n");
> >> +
> >> +unlock:
> >> + mutex_unlock(&q->mmap_lock);
> >> + return ret;
> >> +}
> >> +EXPORT_SYMBOL_GPL(vb2_core_delete_bufs);
> >> +
> >> /*
> >> * vb2_start_streaming() - Attempt to start streaming.
> >> * @q: videobuf2 queue
> >> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> >> index 56daf3b5b2ba..f003618efbc1 100644
> >> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> >> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> >> @@ -694,6 +694,8 @@ static void fill_buf_caps(struct vb2_queue *q, u32 *caps)
> >> if (q->supports_requests)
> >> *caps |= V4L2_BUF_CAP_SUPPORTS_REQUESTS;
> >> #endif
> >> + if (q->supports_delete_bufs)
> >> + *caps |= V4L2_BUF_CAP_SUPPORTS_DELETE_BUFS;
> >> }
> >>
> >> static void validate_memory_flags(struct vb2_queue *q,
> >> @@ -751,6 +753,12 @@ int vb2_prepare_buf(struct vb2_queue *q, struct media_device *mdev,
> >> }
> >> EXPORT_SYMBOL_GPL(vb2_prepare_buf);
> >>
> >> +int vb2_delete_bufs(struct vb2_queue *q, struct v4l2_delete_buffers *d)
> >> +{
> >> + return vb2_core_delete_bufs(q, d->index, d->count);
> >> +}
> >> +EXPORT_SYMBOL_GPL(vb2_delete_bufs);
> >> +
> >> int vb2_create_bufs(struct vb2_queue *q, struct v4l2_create_buffers *create)
> >> {
> >> unsigned requested_planes = 1;
> >> @@ -1012,6 +1020,18 @@ EXPORT_SYMBOL_GPL(vb2_poll);
> >>
> >> /* vb2 ioctl helpers */
> >>
> >> +int vb2_ioctl_delete_bufs(struct file *file, void *priv,
> >> + struct v4l2_delete_buffers *p)
> >> +{
> >> + struct video_device *vdev = video_devdata(file);
> >> +
> >> + if (vb2_queue_is_busy(vdev->queue, file))
> >> + return -EBUSY;
> >> +
> >> + return vb2_delete_bufs(vdev->queue, p);
> >> +}
> >> +EXPORT_SYMBOL_GPL(vb2_ioctl_delete_bufs);
> >> +
> >> int vb2_ioctl_reqbufs(struct file *file, void *priv,
> >> struct v4l2_requestbuffers *p)
> >> {
> >> diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c
> >> index f81279492682..215654fd6581 100644
> >> --- a/drivers/media/v4l2-core/v4l2-dev.c
> >> +++ b/drivers/media/v4l2-core/v4l2-dev.c
> >> @@ -720,6 +720,7 @@ static void determine_valid_ioctls(struct video_device *vdev)
> >> SET_VALID_IOCTL(ops, VIDIOC_PREPARE_BUF, vidioc_prepare_buf);
> >> SET_VALID_IOCTL(ops, VIDIOC_STREAMON, vidioc_streamon);
> >> SET_VALID_IOCTL(ops, VIDIOC_STREAMOFF, vidioc_streamoff);
> >> + SET_VALID_IOCTL(ops, VIDIOC_DELETE_BUFS, vidioc_delete_bufs);
> >> }
> >>
> >> if (is_vid || is_vbi || is_meta) {
> >> diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
> >> index 4d90424cbfc4..e9dcf439f9f3 100644
> >> --- a/drivers/media/v4l2-core/v4l2-ioctl.c
> >> +++ b/drivers/media/v4l2-core/v4l2-ioctl.c
> >> @@ -489,6 +489,13 @@ static void v4l_print_create_buffers(const void *arg, bool write_only)
> >> v4l_print_format(&p->format, write_only);
> >> }
> >>
> >> +static void v4l_print_delete_buffers(const void *arg, bool write_only)
> >> +{
> >> + const struct v4l2_delete_buffers *p = arg;
> >> +
> >> + pr_cont("index=%u, count=%u\n", p->index, p->count);
> >> +}
> >> +
> >> static void v4l_print_streamparm(const void *arg, bool write_only)
> >> {
> >> const struct v4l2_streamparm *p = arg;
> >> @@ -2161,6 +2168,15 @@ static int v4l_prepare_buf(const struct v4l2_ioctl_ops *ops,
> >> return ret ? ret : ops->vidioc_prepare_buf(file, fh, b);
> >> }
> >>
> >> +static int v4l_delete_bufs(const struct v4l2_ioctl_ops *ops,
> >> + struct file *file, void *fh, void *arg)
> >> +{
> >> + struct v4l2_delete_buffers *delete = arg;
> >> + int ret = check_fmt(file, delete->type);
> > Should we also have
> >
> > memset_after(delete, 0, type);
> >
> > here similar to how other ioctl handlers clear the reserved fields?
>
> I will add it.
>
> >
> >> +
> >> + return ret ? ret : ops->vidioc_delete_bufs(file, fh, delete);
> >> +}
> >> +
> >> static int v4l_g_parm(const struct v4l2_ioctl_ops *ops,
> >> struct file *file, void *fh, void *arg)
> >> {
> >> @@ -2910,6 +2926,7 @@ static const struct v4l2_ioctl_info v4l2_ioctls[] = {
> >> IOCTL_INFO(VIDIOC_ENUM_FREQ_BANDS, v4l_enum_freq_bands, v4l_print_freq_band, 0),
> >> IOCTL_INFO(VIDIOC_DBG_G_CHIP_INFO, v4l_dbg_g_chip_info, v4l_print_dbg_chip_info, INFO_FL_CLEAR(v4l2_dbg_chip_info, match)),
> >> IOCTL_INFO(VIDIOC_QUERY_EXT_CTRL, v4l_query_ext_ctrl, v4l_print_query_ext_ctrl, INFO_FL_CTRL | INFO_FL_CLEAR(v4l2_query_ext_ctrl, id)),
> >> + IOCTL_INFO(VIDIOC_DELETE_BUFS, v4l_delete_bufs, v4l_print_delete_buffers, INFO_FL_PRIO | INFO_FL_QUEUE | INFO_FL_CLEAR(v4l2_delete_buffers, type)),
> >> };
> >> #define V4L2_IOCTLS ARRAY_SIZE(v4l2_ioctls)
> >>
> >> diff --git a/include/media/v4l2-ioctl.h b/include/media/v4l2-ioctl.h
> >> index edb733f21604..55afbde54211 100644
> >> --- a/include/media/v4l2-ioctl.h
> >> +++ b/include/media/v4l2-ioctl.h
> >> @@ -163,6 +163,8 @@ struct v4l2_fh;
> >> * :ref:`VIDIOC_CREATE_BUFS <vidioc_create_bufs>` ioctl
> >> * @vidioc_prepare_buf: pointer to the function that implements
> >> * :ref:`VIDIOC_PREPARE_BUF <vidioc_prepare_buf>` ioctl
> >> + * @vidioc_delete_bufs: pointer to the function that implements
> >> + * :ref:`VIDIOC_DELETE_BUFS <vidioc_delete_bufs>` ioctl
> >> * @vidioc_overlay: pointer to the function that implements
> >> * :ref:`VIDIOC_OVERLAY <vidioc_overlay>` ioctl
> >> * @vidioc_g_fbuf: pointer to the function that implements
> >> @@ -422,6 +424,8 @@ struct v4l2_ioctl_ops {
> >> struct v4l2_create_buffers *b);
> >> int (*vidioc_prepare_buf)(struct file *file, void *fh,
> >> struct v4l2_buffer *b);
> >> + int (*vidioc_delete_bufs)(struct file *file, void *fh,
> >> + struct v4l2_delete_buffers *d);
> >>
> >> int (*vidioc_overlay)(struct file *file, void *fh, unsigned int i);
> >> int (*vidioc_g_fbuf)(struct file *file, void *fh,
> >> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> >> index 288477075a0e..db3bd986624e 100644
> >> --- a/include/media/videobuf2-core.h
> >> +++ b/include/media/videobuf2-core.h
> >> @@ -507,6 +507,7 @@ struct vb2_buf_ops {
> >> * @supports_requests: this queue supports the Request API.
> >> * @requires_requests: this queue requires the Request API. If this is set to 1,
> >> * then supports_requests must be set to 1 as well.
> >> + * @supports_delete_bufs: this queue supports DELETE_BUFS ioctl.
> >> * @uses_qbuf: qbuf was used directly for this queue. Set to 1 the first
> >> * time this is called. Set to 0 when the queue is canceled.
> >> * If this is 1, then you cannot queue buffers from a request.
> >> @@ -595,6 +596,7 @@ struct vb2_queue {
> >> unsigned int quirk_poll_must_check_waiting_for_buffers:1;
> >> unsigned int supports_requests:1;
> >> unsigned int requires_requests:1;
> >> + unsigned int supports_delete_bufs:1;
> >> unsigned int uses_qbuf:1;
> >> unsigned int uses_requests:1;
> >> unsigned int allow_cache_hints:1;
> >> @@ -846,6 +848,16 @@ int vb2_core_create_bufs(struct vb2_queue *q, enum vb2_memory memory,
> >> */
> >> int vb2_core_prepare_buf(struct vb2_queue *q, struct vb2_buffer *vb, void *pb);
> >>
> >> +/**
> >> + * vb2_core_delete_bufs() -
> >> + * @q: pointer to &struct vb2_queue with videobuf2 queue.
> >> + * @start: first index of the range of buffers to delete.
> >> + * @count: number of buffers to delete.
> >> + *
> >> + * Return: returns zero on success; an error code otherwise.
> >> + */
> >> +int vb2_core_delete_bufs(struct vb2_queue *q, unsigned int start, unsigned int count);
> >> +
> >> /**
> >> * vb2_core_qbuf() - Queue a buffer from userspace
> >> *
> >> diff --git a/include/media/videobuf2-v4l2.h b/include/media/videobuf2-v4l2.h
> >> index 5a845887850b..79cea8459f52 100644
> >> --- a/include/media/videobuf2-v4l2.h
> >> +++ b/include/media/videobuf2-v4l2.h
> >> @@ -118,6 +118,17 @@ int vb2_create_bufs(struct vb2_queue *q, struct v4l2_create_buffers *create);
> >> */
> >> int vb2_prepare_buf(struct vb2_queue *q, struct media_device *mdev,
> >> struct v4l2_buffer *b);
> >> +/**
> >> + * vb2_delete_bufs() - Delete buffers from the queue
> >> + *
> >> + * @q: pointer to &struct vb2_queue with videobuf2 queue.
> >> + * @d: delete parameter, passed from userspace to
> >> + * &v4l2_ioctl_ops->vidioc_delete_bufs handler in driver
> >> + *
> >> + * The return values from this function are intended to be directly returned
> >> + * from &v4l2_ioctl_ops->vidioc_delete_bufs handler in driver.
> >> + */
> >> +int vb2_delete_bufs(struct vb2_queue *q, struct v4l2_delete_buffers *d);
> >>
> >> /**
> >> * vb2_qbuf() - Queue a buffer from userspace
> >> @@ -334,6 +345,8 @@ int vb2_ioctl_streamon(struct file *file, void *priv, enum v4l2_buf_type i);
> >> int vb2_ioctl_streamoff(struct file *file, void *priv, enum v4l2_buf_type i);
> >> int vb2_ioctl_expbuf(struct file *file, void *priv,
> >> struct v4l2_exportbuffer *p);
> >> +int vb2_ioctl_delete_bufs(struct file *file, void *priv,
> >> + struct v4l2_delete_buffers *p);
> >>
> >> /* struct v4l2_file_operations helpers */
> >>
> >> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> >> index 13ddb5abf584..96e105149906 100644
> >> --- a/include/uapi/linux/videodev2.h
> >> +++ b/include/uapi/linux/videodev2.h
> >> @@ -1036,6 +1036,7 @@ struct v4l2_requestbuffers {
> >> #define V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF (1 << 5)
> >> #define V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS (1 << 6)
> >> #define V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS (1 << 7)
> >> +#define V4L2_BUF_CAP_SUPPORTS_DELETE_BUFS (1 << 8)
> >>
> >> /**
> >> * struct v4l2_plane - plane info for multi-planar buffers
> >> @@ -2622,6 +2623,20 @@ struct v4l2_create_buffers {
> >> __u32 reserved[5];
> >> };
> >>
> >> +/**
> >> + * struct v4l2_delete_buffers - VIDIOC_DELETE_BUFS argument
> >> + * @index: the first buffer to be deleted
> >> + * @count: number of buffers to delete
> >> + * @type: enum v4l2_buf_type
> >> + * @reserved: future extensions
> >> + */
> >> +struct v4l2_delete_buffers {
> >> + __u32 index;
> >> + __u32 count;
> >> + __u32 type;
> >> + __u32 reserved[13];
> >> +};
> >> +
> >> /*
> >> * I O C T L C O D E S F O R V I D E O D E V I C E S
> >> *
> >> @@ -2721,6 +2736,8 @@ struct v4l2_create_buffers {
> >> #define VIDIOC_DBG_G_CHIP_INFO _IOWR('V', 102, struct v4l2_dbg_chip_info)
> >>
> >> #define VIDIOC_QUERY_EXT_CTRL _IOWR('V', 103, struct v4l2_query_ext_ctrl)
> >> +#define VIDIOC_DELETE_BUFS _IOWR('V', 104, struct v4l2_delete_buffers)
> >> +
> >>
> >> /* Reminder: when adding new ioctls please add support for them to
> >> drivers/media/v4l2-core/v4l2-compat-ioctl32.c as well! */
> > Do we also need to add support for the new ioctl in there?
>
> It only use _u32, is aligned on cache boundary and doesn't use pointer.
> For me 32 and 64 bits structures are identical so it doesn't need to be convert.
> Maybe I'm wrong when testing it with pahole ?
Ah, right. I double checked the compat code and indeed if an ioctl is
not listed there, it will just use the default copy_from_user, so we
should be good.
Maybe one day we should update that comment to explain how to check if
it's necessary to add a compat handler.
Best regards,
Tomasz
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