Re: [PATCH v14 08/56] media: videobuf2: Use vb2_get_num_buffers() helper

[Tomasz Figa]

On Wed, Nov 8, 2023 at 10:22 PM Benjamin Gaignard
<benjamin.gaignard@xxxxxxxxxxxxx> wrote:
>
>
> Le 08/11/2023 à 10:42, Tomasz Figa a écrit :
> > On Tue, Oct 31, 2023 at 05:30:16PM +0100, Benjamin Gaignard wrote:
> >> Stop using queue num_buffers field directly, instead use
> >> vb2_get_num_buffers().
> >> This prepares for the future 'delete buffers' feature where there are
> >> holes in the buffer indices.
> >>
> >> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx>
> >> ---
> >>   .../media/common/videobuf2/videobuf2-core.c   | 92 +++++++++++--------
> >>   .../media/common/videobuf2/videobuf2-v4l2.c   |  4 +-
> >>   2 files changed, 54 insertions(+), 42 deletions(-)
> >>
> >> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> >> index b406a30a9b35..c5c5ae4d213d 100644
> >> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> >> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> >> @@ -444,13 +444,14 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
> >>                           unsigned int num_buffers, unsigned int num_planes,
> >>                           const unsigned plane_sizes[VB2_MAX_PLANES])
> >>   {
> >> +    unsigned int q_num_buffers = vb2_get_num_buffers(q);
> >>      unsigned int buffer, plane;
> >>      struct vb2_buffer *vb;
> >>      int ret;
> >>
> >>      /* Ensure that q->num_buffers+num_buffers is below VB2_MAX_FRAME */
> >>      num_buffers = min_t(unsigned int, num_buffers,
> >> -                        VB2_MAX_FRAME - q->num_buffers);
> >> +                        VB2_MAX_FRAME - q_num_buffers);
> > I guess it's safe in this specific situation, but was there any reason
> > behind not just calling vb2_get_num_buffers() directly here?
> >
> >>
> >>      for (buffer = 0; buffer < num_buffers; ++buffer) {
> >>              /* Allocate vb2 buffer structures */
> >> @@ -470,7 +471,7 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
> >>                      vb->planes[plane].min_length = plane_sizes[plane];
> >>              }
> >>
> >> -            vb2_queue_add_buffer(q, vb, q->num_buffers + buffer);
> >> +            vb2_queue_add_buffer(q, vb, q_num_buffers + buffer);
> > In this case it should also be fine, but actually now this is a loop and if
> > somone doesn't know what the other code in the loop does, one could be
> > concerned that the num buffers actually could have changed, but we still
> > use the cached one that we got at the beginning of the function.
> >
> > (Ideally I'd imagine vb2_queue_add_buffer() to append the buffer
> > at the end of the queue and increment the num_buffers internally, but it
> > doesn't have to happen now, as this series is already quite complex...)
>
> That will be the case later in the series when I replace num_buffers field
> by a bitmap. Until that I prefer to limit the changes in this loop.
>
> >
> >>              call_void_bufop(q, init_buffer, vb);
> >>
> >>              /* Allocate video buffer memory for the MMAP type */
> > [snip]
> >> @@ -2513,7 +2519,8 @@ void vb2_core_queue_release(struct vb2_queue *q)
> >>      __vb2_cleanup_fileio(q);
> >>      __vb2_queue_cancel(q);
> >>      mutex_lock(&q->mmap_lock);
> >> -    __vb2_queue_free(q, q->num_buffers);
> >> +    __vb2_queue_free(q, vb2_get_num_buffers(q));
> >> +    q->num_buffers = 0;
> > Unrelated change?
>
> No because I found a case where q->num_buffers wasn't correctly reset while testing.
>

Could you provide more details about that case? Shouldn't it be fixed instead?

It's a bit weird to me, because __vb2_queue_free() is supposed to
decrement q->num_buffers by the number of buffers freed and it's
called with vb2_get_num_buffers() just one line above.

> >
> >>      mutex_unlock(&q->mmap_lock);
> >>   }
> >>   EXPORT_SYMBOL_GPL(vb2_core_queue_release);
> >> @@ -2542,7 +2549,7 @@ __poll_t vb2_core_poll(struct vb2_queue *q, struct file *file,
> >>      /*
> >>       * Start file I/O emulator only if streaming API has not been used yet.
> >>       */
> >> -    if (q->num_buffers == 0 && !vb2_fileio_is_active(q)) {
> >> +    if (vb2_get_num_buffers(q) == 0 && !vb2_fileio_is_active(q)) {
> >>              if (!q->is_output && (q->io_modes & VB2_READ) &&
> >>                              (req_events & (EPOLLIN | EPOLLRDNORM))) {
> >>                      if (__vb2_init_fileio(q, 1))
> >> @@ -2580,7 +2587,7 @@ __poll_t vb2_core_poll(struct vb2_queue *q, struct file *file,
> >>       * For output streams you can call write() as long as there are fewer
> >>       * buffers queued than there are buffers available.
> >>       */
> >> -    if (q->is_output && q->fileio && q->queued_count < q->num_buffers)
> >> +    if (q->is_output && q->fileio && q->queued_count < vb2_get_num_buffers(q))
> >>              return EPOLLOUT | EPOLLWRNORM;
> >>
> >>      if (list_empty(&q->done_list)) {
> >> @@ -2629,8 +2636,8 @@ struct vb2_fileio_buf {
> >>    * struct vb2_fileio_data - queue context used by file io emulator
> >>    *
> >>    * @cur_index:     the index of the buffer currently being read from or
> >> - *          written to. If equal to q->num_buffers then a new buffer
> >> - *          must be dequeued.
> >> + *          written to. If equal to number of already queued buffers
> >> + *          then a new buffer must be dequeued.
> > Hmm, that's a significant meaning change compared to the original text. Is
> > it indended?
>
> Does "If equal to number of buffers in the vb2_queue then a new buffer must be dequeued."
> sound better for you ?

Yes, I think now it matches the original meaning. Thanks.

>
> >
> >>    * @initial_index: in the read() case all buffers are queued up immediately
> >>    *         in __vb2_init_fileio() and __vb2_perform_fileio() just cycles
> >>    *         buffers. However, in the write() case no buffers are initially
> >> @@ -2640,7 +2647,7 @@ struct vb2_fileio_buf {
> >>    *         buffers. This means that initially __vb2_perform_fileio()
> >>    *         needs to know what buffer index to use when it is queuing up
> >>    *         the buffers for the first time. That initial index is stored
> >> - *          in this field. Once it is equal to q->num_buffers all
> >> + *          in this field. Once it is equal to num_buffers all
> > It's not clear what num_buffers means here. Would it make sense to instead
> > say "number of buffers in the vb2_queue"?
>
> Yes I will change that
>
> >
> >>    *         available buffers have been queued and __vb2_perform_fileio()
> >>    *         should start the normal dequeue/queue cycle.
> >>    *
> >> @@ -2690,7 +2697,7 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
> >>      /*
> >>       * Check if streaming api has not been already activated.
> >>       */
> >> -    if (q->streaming || q->num_buffers > 0)
> >> +    if (q->streaming || vb2_get_num_buffers(q) > 0)
> >>              return -EBUSY;
> >>
> >>      /*
> >> @@ -2740,7 +2747,7 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
> >>      /*
> >>       * Get kernel address of each buffer.
> >>       */
> >> -    for (i = 0; i < q->num_buffers; i++) {
> >> +    for (i = 0; i < vb2_get_num_buffers(q); i++) {
> >>              /* vb can never be NULL when using fileio. */
> >>              vb = vb2_get_buffer(q, i);
> >>
> >> @@ -2759,18 +2766,23 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
> >>              /*
> >>               * Queue all buffers.
> >>               */
> >> -            for (i = 0; i < q->num_buffers; i++) {
> >> -                    ret = vb2_core_qbuf(q, q->bufs[i], NULL, NULL);
> >> +            for (i = 0; i < vb2_get_num_buffers(q); i++) {
> >> +                    struct vb2_buffer *vb2 = vb2_get_buffer(q, i);
> >> +
> >> +                    if (!vb2)
> >> +                            continue;
> >> +
> >> +                    ret = vb2_core_qbuf(q, vb2, NULL, NULL);
> >>                      if (ret)
> >>                              goto err_reqbufs;
> >>                      fileio->bufs[i].queued = 1;
> >>              }
> > Doesn't this part belong to the previous patch that changes q->bufs[x] to
> > vb2_get_buffer()?
>
> Yes I will change that too.
>
> >
> >>              /*
> >>               * All buffers have been queued, so mark that by setting
> >> -             * initial_index to q->num_buffers
> >> +             * initial_index to num_buffers
> > What num_buffers?
>
> I will use your wording: "the number of buffers in the vb2_queue"
>

Thanks!

> >
> >>               */
> >> -            fileio->initial_index = q->num_buffers;
> >> -            fileio->cur_index = q->num_buffers;
> >> +            fileio->initial_index = vb2_get_num_buffers(q);
> >> +            fileio->cur_index = fileio->initial_index;
> >>      }
> >>
> >>      /*
> >> @@ -2964,12 +2976,12 @@ static size_t __vb2_perform_fileio(struct vb2_queue *q, char __user *data, size_
> >>               * If we are queuing up buffers for the first time, then
> >>               * increase initial_index by one.
> >>               */
> >> -            if (fileio->initial_index < q->num_buffers)
> >> +            if (fileio->initial_index < vb2_get_num_buffers(q))
> >>                      fileio->initial_index++;
> >>              /*
> >>               * The next buffer to use is either a buffer that's going to be
> >> -             * queued for the first time (initial_index < q->num_buffers)
> >> -             * or it is equal to q->num_buffers, meaning that the next
> >> +             * queued for the first time (initial_index < num_buffers)
> >> +             * or it is equal to num_buffers, meaning that the next
> > What num_buffers?
>
> Same here

Thanks!

Best regards,
Tomasz