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SubjectRe: [RFC v5 07/11] [media] vb2: add in-fence support to QBUF
2017-11-17 Alexandre Courbot <acourbot@chromium.org>:

> Hi Gustavo,
>
> I am coming a bit late in this series' review, so apologies if some of my
> comments have already have been discussed in an earlier revision.
>
> On Thursday, November 16, 2017 2:10:53 AM JST, Gustavo Padovan wrote:
> > From: Gustavo Padovan <gustavo.padovan@collabora.com>
> >
> > Receive in-fence from userspace and add support for waiting on them
> > before queueing the buffer to the driver. Buffers can't be queued to the
> > driver before its fences signal. And a buffer can't be queue to the driver
> > out of the order they were queued from userspace. That means that even if
> > it fence signal it must wait all other buffers, ahead of it in the queue,
> > to signal first.
> >
> > To make that possible we use fence_array to keep that ordering. Basically
> > we create a fence_array that contains both the current fence and the fence
> > from the previous buffer (which might be a fence array as well). The base
> > fence class for the fence_array becomes the new buffer fence, waiting on
> > that one guarantees that it won't be queued out of order.
> >
> > v6:
> > - With fences always keep the order userspace queues the buffers.
> > - Protect in_fence manipulation with a lock (Brian Starkey)
> > - check if fences have the same context before adding a fence array
> > - Fix last_fence ref unbalance in __set_in_fence() (Brian Starkey)
> > - Clean up fence if __set_in_fence() fails (Brian Starkey)
> > - treat -EINVAL from dma_fence_add_callback() (Brian Starkey)
> >
> > v5: - use fence_array to keep buffers ordered in vb2 core when
> > needed (Brian Starkey)
> > - keep backward compat on the reserved2 field (Brian Starkey)
> > - protect fence callback removal with lock (Brian Starkey)
> >
> > v4:
> > - Add a comment about dma_fence_add_callback() not returning a
> > error (Hans)
> > - Call dma_fence_put(vb->in_fence) if fence signaled (Hans)
> > - select SYNC_FILE under config VIDEOBUF2_CORE (Hans)
> > - Move dma_fence_is_signaled() check to __enqueue_in_driver() (Hans)
> > - Remove list_for_each_entry() in __vb2_core_qbuf() (Hans)
> > - Remove if (vb->state != VB2_BUF_STATE_QUEUED) from
> > vb2_start_streaming() (Hans)
> > - set IN_FENCE flags on __fill_v4l2_buffer (Hans)
> > - Queue buffers to the driver as soon as they are ready (Hans)
> > - call fill_user_buffer() after queuing the buffer (Hans)
> > - add err: label to clean up fence
> > - add dma_fence_wait() before calling vb2_start_streaming()
> >
> > v3: - document fence parameter
> > - remove ternary if at vb2_qbuf() return (Mauro)
> > - do not change if conditions behaviour (Mauro)
> >
> > v2:
> > - fix vb2_queue_or_prepare_buf() ret check
> > - remove check for VB2_MEMORY_DMABUF only (Javier)
> > - check num of ready buffers to start streaming
> > - when queueing, start from the first ready buffer
> > - handle queue cancel
> >
> > Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
> > ---
> > drivers/media/v4l2-core/Kconfig | 1 +
> > drivers/media/v4l2-core/videobuf2-core.c | 202
> > ++++++++++++++++++++++++++++---
> > drivers/media/v4l2-core/videobuf2-v4l2.c | 29 ++++-
> > include/media/videobuf2-core.h | 17 ++-
> > 4 files changed, 231 insertions(+), 18 deletions(-)
> >
> > diff --git a/drivers/media/v4l2-core/Kconfig
> > b/drivers/media/v4l2-core/Kconfig
> > index a35c33686abf..3f988c407c80 100644
> > --- a/drivers/media/v4l2-core/Kconfig
> > +++ b/drivers/media/v4l2-core/Kconfig
> > @@ -83,6 +83,7 @@ config VIDEOBUF_DVB
> > # Used by drivers that need Videobuf2 modules
> > config VIDEOBUF2_CORE
> > select DMA_SHARED_BUFFER
> > + select SYNC_FILE
> > tristate
> > config VIDEOBUF2_MEMOPS
> > diff --git a/drivers/media/v4l2-core/videobuf2-core.c
> > b/drivers/media/v4l2-core/videobuf2-core.c
> > index 60f8b582396a..26de4c80717d 100644
> > --- a/drivers/media/v4l2-core/videobuf2-core.c
> > +++ b/drivers/media/v4l2-core/videobuf2-core.c
> > @@ -23,6 +23,7 @@
> > #include <linux/sched.h>
> > #include <linux/freezer.h>
> > #include <linux/kthread.h>
> > +#include <linux/dma-fence-array.h>
> > #include <media/videobuf2-core.h>
> > #include <media/v4l2-mc.h>
> > @@ -346,6 +347,7 @@ static int __vb2_queue_alloc(struct vb2_queue *q,
> > enum vb2_memory memory,
> > vb->index = q->num_buffers + buffer;
> > vb->type = q->type;
> > vb->memory = memory;
> > + spin_lock_init(&vb->fence_cb_lock);
> > for (plane = 0; plane < num_planes; ++plane) {
> > vb->planes[plane].length = plane_sizes[plane];
> > vb->planes[plane].min_length = plane_sizes[plane];
> > @@ -1222,6 +1224,9 @@ static void __enqueue_in_driver(struct vb2_buffer *vb)
> > {
> > struct vb2_queue *q = vb->vb2_queue;
> > + if (vb->in_fence && !dma_fence_is_signaled(vb->in_fence))
> > + return;
> > +
> > vb->state = VB2_BUF_STATE_ACTIVE;
> > atomic_inc(&q->owned_by_drv_count);
> > @@ -1273,6 +1278,23 @@ static int __buf_prepare(struct vb2_buffer *vb,
> > const void *pb)
> > return 0;
> > }
> > +static int __get_num_ready_buffers(struct vb2_queue *q)
> > +{
> > + struct vb2_buffer *vb;
> > + int ready_count = 0;
> > + unsigned long flags;
> > +
> > + /* count num of buffers ready in front of the queued_list */
> > + list_for_each_entry(vb, &q->queued_list, queued_entry) {
> > + spin_lock_irqsave(&vb->fence_cb_lock, flags);
> > + if (!vb->in_fence || dma_fence_is_signaled(vb->in_fence))
> > + ready_count++;
> > + spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> > + }
> > +
> > + return ready_count;
> > +}
> > +
> > int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb)
> > {
> > struct vb2_buffer *vb;
> > @@ -1361,9 +1383,87 @@ static int vb2_start_streaming(struct vb2_queue *q)
> > return ret;
> > }
> > -int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb)
> > +static struct dma_fence *__set_in_fence(struct vb2_queue *q,
> > + struct vb2_buffer *vb,
> > + struct dma_fence *fence)
> > +{
> > + if (q->last_fence && dma_fence_is_signaled(q->last_fence)) {
> > + dma_fence_put(q->last_fence);
> > + q->last_fence = NULL;
> > + }
> > +
> > + /*
> > + * We always guarantee the ordering of buffers queued from
> > + * userspace to be the same it is queued to the driver. For that
> > + * we create a fence array with the fence from the last queued
> > + * buffer and this one, that way the fence for this buffer can't
> > + * signal before the last one.
> > + */
> > + if (fence && q->last_fence) {
> > + struct dma_fence **fences;
> > + struct dma_fence_array *arr;
> > +
> > + if (fence->context == q->last_fence->context) {
> > + if (fence->seqno - q->last_fence->seqno <= INT_MAX) {
> > + dma_fence_put(q->last_fence);
> > + q->last_fence = dma_fence_get(fence);
> > + } else {
> > + dma_fence_put(fence);
> > + fence = dma_fence_get(q->last_fence);
> > + }
> > + return fence;
> > + }
> > +
> > + fences = kcalloc(2, sizeof(*fences), GFP_KERNEL);
> > + if (!fences)
> > + return ERR_PTR(-ENOMEM);
> > +
> > + fences[0] = fence;
> > + fences[1] = q->last_fence;
> > +
> > + arr = dma_fence_array_create(2, fences,
> > + dma_fence_context_alloc(1),
> > + 1, false);
> > + if (!arr) {
> > + kfree(fences);
> > + return ERR_PTR(-ENOMEM);
> > + }
> > +
> > + fence = &arr->base;
> > +
> > + q->last_fence = dma_fence_get(fence);
> > + } else if (!fence && q->last_fence) {
> > + fence = dma_fence_get(q->last_fence);
> > + }
> > +
> > + return fence;
> > +}
> > +
> > +static void vb2_qbuf_fence_cb(struct dma_fence *f, struct dma_fence_cb *cb)
> > +{
> > + struct vb2_buffer *vb = container_of(cb, struct vb2_buffer, fence_cb);
> > + struct vb2_queue *q = vb->vb2_queue;
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(&vb->fence_cb_lock, flags);
> > + if (!vb->in_fence) {
> > + spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> > + return;
> > + }
>
> Is this block necessary? IIUC the callback will never be set on buffers
> without
> an input fence, so (!vb->in_fence) should never be satisfied.

Not anymore! I added it when trying to fix the potential race condition
in the wrong way, but the newly added spinlock fixes that.

>
> > +
> > + dma_fence_put(vb->in_fence);
> > + vb->in_fence = NULL;
> > +
> > + if (q->start_streaming_called)
> > + __enqueue_in_driver(vb);
> > + spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> > +}
> > +
> > +int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
> > + struct dma_fence *fence)
> > {
> > struct vb2_buffer *vb;
> > + unsigned long flags;
> > int ret;
> > vb = q->bufs[index];
> > @@ -1372,16 +1472,18 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned
> > int index, void *pb)
> > case VB2_BUF_STATE_DEQUEUED:
> > ret = __buf_prepare(vb, pb);
> > if (ret)
> > - return ret;
> > + goto err;
> > break;
> > case VB2_BUF_STATE_PREPARED:
> > break;
> > case VB2_BUF_STATE_PREPARING:
> > dprintk(1, "buffer still being prepared\n");
> > - return -EINVAL;
> > + ret = -EINVAL;
> > + goto err;
> > default:
> > dprintk(1, "invalid buffer state %d\n", vb->state);
> > - return -EINVAL;
> > + ret = -EINVAL;
> > + goto err;
> > }
> > /*
> > @@ -1398,30 +1500,83 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned
> > int index, void *pb)
> > trace_vb2_qbuf(q, vb);
> > + vb->in_fence = __set_in_fence(q, vb, fence);
> > + if (IS_ERR(vb->in_fence)) {
> > + dma_fence_put(fence);
> > + ret = PTR_ERR(vb->in_fence);
> > + goto err;
> > + }
> > + fence = NULL;
> > +
> > + /*
> > + * If it is time to call vb2_start_streaming() wait for the fence
> > + * to signal first. Of course, this happens only once per streaming.
> > + * We want to run any step that might fail before we set the callback
> > + * to queue the fence when it signals.
> > + */
> > + if (vb->in_fence && !q->start_streaming_called &&
> > + __get_num_ready_buffers(q) == q->min_buffers_needed - 1)
> > + dma_fence_wait(vb->in_fence, true);
>
> Mmm, that's a tough call. Userspace may unexpectingly block due to this
> (which
> fences are supposed to prevent), not sure how much of a problem this may be.

Yes, but it happen just once per stream, when we are about to start it.
Alternatively, we can wait later, and if the fence wait returns a error
we can carry it until we are able to send the buffer back with error on
DQBUF.

>
> > +
> > /*
> > * If streamon has been called, and we haven't yet called
> > * start_streaming() since not enough buffers were queued, and
> > * we now have reached the minimum number of queued buffers,
> > * then we can finally call start_streaming().
> > - *
> > - * If already streaming, give the buffer to driver for processing.
> > - * If not, the buffer will be given to driver on next streamon.
> > */
> > if (q->streaming && !q->start_streaming_called &&
> > - q->queued_count >= q->min_buffers_needed) {
> > + __get_num_ready_buffers(q) >= q->min_buffers_needed) {
> > ret = vb2_start_streaming(q);
> > if (ret)
> > - return ret;
> > - } else if (q->start_streaming_called) {
> > - __enqueue_in_driver(vb);
> > + goto err;
> > }
> > + /*
> > + * For explicit synchronization: If the fence didn't signal
> > + * yet we setup a callback to queue the buffer once the fence
> > + * signals, and then, return successfully. But if the fence
> > + * already signaled we lose the reference we held and queue the
> > + * buffer to the driver.
> > + */
> > + spin_lock_irqsave(&vb->fence_cb_lock, flags);
> > + if (vb->in_fence) {
> > + ret = dma_fence_add_callback(vb->in_fence, &vb->fence_cb,
> > + vb2_qbuf_fence_cb);
> > + if (ret == -EINVAL) {
> > + spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> > + goto err;
> > + } else if (!ret) {
> > + goto fill;
> > + }
> > +
> > + dma_fence_put(vb->in_fence);
> > + vb->in_fence = NULL;
>
> I suppose the last two lines are supposed to be called when
> dma_fence_add_callback() returns -ENOENT, because the fence has already been
> signaled? In that case I think the code would be more readable if you made
> it
> more explicit, something like:
>
> // fence already signaled?
> if (ret == -ENOENT) {
> dma_fence_put(vb->in_fence);
> vb->in_fence = NULL;
> } else if (ret != 0) {
> spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> goto err;
> }
>
> and that way you can get rid of the short-ranged fill: label.

Yes, that is a better way to write it!

>
> > + }
> > +
> > +fill:
> > + /*
> > + * If already streaming and there is no fence to wait on
> > + * give the buffer to driver for processing.
> > + */
> > + if (q->start_streaming_called && !vb->in_fence)
> > + __enqueue_in_driver(vb);
>
> Since that code was previously called when vb2_start_streaming() wasn't, we
> had a guarantee that the buffer was only queued once. Now I wonder if we
> could
> not run into a situation where __enqueue_in_driver is called twice for the
> last
> queued buffer, once by vb2_start_streaming() and once here, now that
> vb2_start_streaming() has set q->start_streaming_called to 1?

maybe check for buffer state == QUEUED may suffice, I'll investigate it
further.

>
> > + spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> > +
> > /* Fill buffer information for the userspace */
> > if (pb)
> > call_void_bufop(q, fill_user_buffer, vb, pb);
> > dprintk(2, "qbuf of buffer %d succeeded\n", vb->index);
> > return 0;
> > +
> > +err:
> > + if (vb->in_fence) {
> > + dma_fence_put(vb->in_fence);
> > + vb->in_fence = NULL;
> > + }
> > +
> > + return ret;
> > +
> > }
> > EXPORT_SYMBOL_GPL(vb2_core_qbuf);
> > @@ -1632,6 +1787,8 @@ EXPORT_SYMBOL_GPL(vb2_core_dqbuf);
> > static void __vb2_queue_cancel(struct vb2_queue *q)
> > {
> > unsigned int i;
> > + struct vb2_buffer *vb;
> > + unsigned long flags;
> > /*
> > * Tell driver to stop all transactions and release all queued
> > @@ -1659,6 +1816,21 @@ static void __vb2_queue_cancel(struct vb2_queue *q)
> > q->queued_count = 0;
> > q->error = 0;
> > + list_for_each_entry(vb, &q->queued_list, queued_entry) {
> > + spin_lock_irqsave(&vb->fence_cb_lock, flags);
> > + if (vb->in_fence) {
> > + dma_fence_remove_callback(vb->in_fence, &vb->fence_cb);
> > + dma_fence_put(vb->in_fence);
> > + vb->in_fence = NULL;
> > + }
> > + spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> > + }
> > +
> > + if (q->last_fence) {
> > + dma_fence_put(q->last_fence);
> > + q->last_fence = NULL;
> > + }
> > +
> > /*
> > * Remove all buffers from videobuf's list...
> > */
> > @@ -1720,7 +1892,7 @@ int vb2_core_streamon(struct vb2_queue *q,
> > unsigned int type)
> > * Tell driver to start streaming provided sufficient buffers
> > * are available.
> > */
> > - if (q->queued_count >= q->min_buffers_needed) {
> > + if (__get_num_ready_buffers(q) >= q->min_buffers_needed) {
> > ret = v4l_vb2q_enable_media_source(q);
> > if (ret)
> > return ret;
> > @@ -2240,7 +2412,7 @@ 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, i, NULL);
> > + ret = vb2_core_qbuf(q, i, NULL, NULL);
> > if (ret)
> > goto err_reqbufs;
> > fileio->bufs[i].queued = 1;
> > @@ -2419,7 +2591,7 @@ static size_t __vb2_perform_fileio(struct
> > vb2_queue *q, char __user *data, size_
> > if (copy_timestamp)
> > b->timestamp = ktime_get_ns();
> > - ret = vb2_core_qbuf(q, index, NULL);
> > + ret = vb2_core_qbuf(q, index, NULL, NULL);
> > dprintk(5, "vb2_dbuf result: %d\n", ret);
> > if (ret)
> > return ret;
> > @@ -2522,7 +2694,7 @@ static int vb2_thread(void *data)
> > if (copy_timestamp)
> > vb->timestamp = ktime_get_ns();;
> > if (!threadio->stop)
> > - ret = vb2_core_qbuf(q, vb->index, NULL);
> > + ret = vb2_core_qbuf(q, vb->index, NULL, NULL);
> > call_void_qop(q, wait_prepare, q);
> > if (ret || threadio->stop)
> > break;
> > diff --git a/drivers/media/v4l2-core/videobuf2-v4l2.c
> > b/drivers/media/v4l2-core/videobuf2-v4l2.c
> > index 110fb45fef6f..4c09ea007d90 100644
> > --- a/drivers/media/v4l2-core/videobuf2-v4l2.c
> > +++ b/drivers/media/v4l2-core/videobuf2-v4l2.c
> > @@ -23,6 +23,7 @@
> > #include <linux/sched.h>
> > #include <linux/freezer.h>
> > #include <linux/kthread.h>
> > +#include <linux/sync_file.h>
> > #include <media/v4l2-dev.h>
> > #include <media/v4l2-fh.h>
> > @@ -178,6 +179,12 @@ static int vb2_queue_or_prepare_buf(struct
> > vb2_queue *q, struct v4l2_buffer *b,
> > return -EINVAL;
> > }
> > + if ((b->fence_fd != 0 && b->fence_fd != -1) &&
>
> Why do we need to consider both values invalid? Can 0 ever be a valid fence
> fd?

For backward compatibility we need to exclude 0.

>
> For completeness you may also want to check the case where (b->fence_fd ==
> -1 &&
> (b->flags & V4L2_BUF_FLAG_IN_FENCE)) and return an error ahead of time, but
> I
> suppose the call to sync_file_get_fence() later on will catch this as well.

Yes, I think we can add this check too.

Gustavo

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