Re: [RFC v5 07/11] [media] vb2: add in-fence support to QBUF

From: Alexandre Courbot
Date: Fri Nov 17 2017 - 01:49:49 EST

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@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
---
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.

+
+ 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.

+
/*
* 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.

+ }
+
+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?

+ 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 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.

+ !(b->flags & V4L2_BUF_FLAG_IN_FENCE)) {
+ dprintk(1, "%s: fence_fd set without IN_FENCE flag\n", opname);
+ return -EINVAL;
+ }
+
return __verify_planes_array(q->bufs[b->index], b);
}
@@ -203,9 +210,14 @@ static void __fill_v4l2_buffer(struct vb2_buffer *vb, void *pb)
b->timestamp = ns_to_timeval(vb->timestamp);
b->timecode = vbuf->timecode;
b->sequence = vbuf->sequence;
- b->fence_fd = -1;
b->reserved = 0;
+ b->fence_fd = -1;
+ if (vb->in_fence)
+ b->flags |= V4L2_BUF_FLAG_IN_FENCE;
+ else
+ b->flags &= ~V4L2_BUF_FLAG_IN_FENCE;
+
if (q->is_multiplanar) {
/*
* Fill in plane-related data if userspace provided an array
@@ -560,6 +572,7 @@ EXPORT_SYMBOL_GPL(vb2_create_bufs);
int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
{
+ struct dma_fence *fence = NULL;
int ret;
if (vb2_fileio_is_active(q)) {
@@ -568,7 +581,19 @@ int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
}
ret = vb2_queue_or_prepare_buf(q, b, "qbuf");
- return ret ? ret : vb2_core_qbuf(q, b->index, b);
+ if (ret)
+ return ret;
+
+ if (b->flags & V4L2_BUF_FLAG_IN_FENCE) {
+ fence = sync_file_get_fence(b->fence_fd);
+ if (!fence) {
+ dprintk(1, "failed to get in-fence from fd %d\n",
+ b->fence_fd);
+ return -EINVAL;
+ }
+ }
+
+ return vb2_core_qbuf(q, b->index, b, fence);
}
EXPORT_SYMBOL_GPL(vb2_qbuf);
diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 38b9c8dd42c6..5f48c7be7770 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -16,6 +16,7 @@
#include <linux/mutex.h>
#include <linux/poll.h>
#include <linux/dma-buf.h>
+#include <linux/dma-fence.h>
#define VB2_MAX_FRAME (32)
#define VB2_MAX_PLANES (8)
@@ -254,11 +255,20 @@ struct vb2_buffer {
* all buffers queued from userspace
* done_entry: entry on the list that stores all buffers ready
* to be dequeued to userspace
+ * in_fence: fence receive from vb2 client to wait on before
+ * using the buffer (queueing to the driver)
+ * fence_cb: fence callback information
+ * fence_cb_lock: protect callback signal/remove
*/
enum vb2_buffer_state state;
struct list_head queued_entry;
struct list_head done_entry;
+
+ struct dma_fence *in_fence;
+ struct dma_fence_cb fence_cb;
+ spinlock_t fence_cb_lock;
+
#ifdef CONFIG_VIDEO_ADV_DEBUG
/*
* Counters for how often these buffer-related ops are
@@ -504,6 +514,7 @@ struct vb2_buf_ops {
* @last_buffer_dequeued: used in poll() and DQBUF to immediately return if the
* last decoded buffer was already dequeued. Set for capture queues
* when a buffer with the V4L2_BUF_FLAG_LAST is dequeued.
+ * @last_fence: last in-fence received. Used to keep ordering.
* @fileio: file io emulator internal data, used only if emulator is active
* @threadio: thread io internal data, used only if thread is active
*/
@@ -558,6 +569,8 @@ struct vb2_queue {
unsigned int copy_timestamp:1;
unsigned int last_buffer_dequeued:1;
+ struct dma_fence *last_fence;
+
struct vb2_fileio_data *fileio;
struct vb2_threadio_data *threadio;
@@ -733,6 +746,7 @@ int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb);
* @index: id number of the buffer
* @pb: buffer structure passed from userspace to vidioc_qbuf handler
* in driver
+ * @fence: in-fence to wait on before queueing the buffer
*
* Should be called from vidioc_qbuf ioctl handler of a driver.
* The passed buffer should have been verified.
@@ -747,7 +761,8 @@ int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb);
* The return values from this function are intended to be directly returned
* from vidioc_qbuf handler in driver.
*/
-int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb);
+int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
+ struct dma_fence *fence);
/**
* vb2_core_dqbuf() - Dequeue a buffer to the userspace