[media] vb2: only call start_streaming if sufficient buffers are queued

In commit 02f142ecd24aaf891324ffba8527284c1731b561 support was added to
start_streaming to return -ENOBUFS if insufficient buffers were queued
for the DMA engine to start. The vb2 core would attempt calling
start_streaming again if another buffer would be queued up.

Later analysis uncovered problems with the queue management if start_streaming
would return an error: the buffers are enqueued to the driver before the
start_streaming op is called, so after an error they are never returned to
the vb2 core. The solution for this is to let the driver return them to
the vb2 core in case of an error while starting the DMA engine. However,
in the case of -ENOBUFS that would be weird: it is not a real error, it
just says that more buffers are needed. Requiring start_streaming to give
them back only to have them requeued again the next time the application
calls QBUF is inefficient.

This patch changes this mechanism: it adds a 'min_buffers_needed' field
to vb2_queue that drivers can set with the minimum number of buffers
required to start the DMA engine. The start_streaming op is only called
if enough buffers are queued. The -ENOBUFS handling has been dropped in
favor of this new method.

Drivers are expected to return buffers back to vb2 core with state QUEUED
if start_streaming would return an error. The vb2 core checks for this
and produces a warning if that didn't happen and it will forcefully
reclaim such buffers to ensure that the internal vb2 core state remains
consistent and all buffer-related resources have been correctly freed
and all op calls have been balanced.

__reqbufs() has been updated to check that at least min_buffers_needed
buffers could be allocated. If fewer buffers were allocated then __reqbufs
will free what was allocated and return -ENOMEM. Based on a suggestion from
Pawel Osciak.

__create_bufs() doesn't do that check, since the use of __create_bufs
assumes some advance scenario where the user might want more control.
Instead streamon will check if enough buffers were allocated to prevent
streaming with fewer than the minimum required number of buffers.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

1 files changed, 10 insertions, 4 deletions

diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 36e3e8e2d457..af4621109726 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -356,20 +356,24 @@ struct v4l2_fh;
  * @gfp_flags:  additional gfp flags used when allocating the buffers.
  *              Typically this is 0, but it may be e.g. GFP_DMA or __GFP_DMA32
  *              to force the buffer allocation to a specific memory zone.
+ * @min_buffers_needed: the minimum number of buffers needed before
+ *              start_streaming() can be called. Used when a DMA engine
+ *              cannot be started unless at least this number of buffers
+ *              have been queued into the driver.
  *
  * @memory:     current memory type used
  * @bufs:       videobuf buffer structures
  * @num_buffers: number of allocated/used buffers
  * @queued_list: list of buffers currently queued from userspace
+ * @queued_count: number of buffers queued and ready for streaming.
  * @owned_by_drv_count: number of buffers owned by the driver
  * @done_list:  list of buffers ready to be dequeued to userspace
  * @done_lock:  lock to protect done_list list
  * @done_wq:    waitqueue for processes waiting for buffers ready to be dequeued
  * @alloc_ctx:  memory type/allocator-specific contexts for each plane
  * @streaming:  current streaming state
- * @retry_start_streaming: start_streaming() was called, but there were not enough
- *              buffers queued. If set, then retry calling start_streaming when
- *              queuing a new buffer.
+ * @start_streaming_called: start_streaming() was called successfully and we
+ *              started streaming.
  * @fileio:     file io emulator internal data, used only if emulator is active
  */
 struct vb2_queue {
@@ -385,6 +389,7 @@ struct vb2_queue {
         unsigned int                    buf_struct_size;
         u32                             timestamp_flags;
         gfp_t                           gfp_flags;
+        u32                             min_buffers_needed;
 
 /* private: internal use only */
         enum v4l2_memory                memory;
@@ -392,6 +397,7 @@ struct vb2_queue {
         unsigned int                    num_buffers;
 
         struct list_head                queued_list;
+        unsigned int                    queued_count;
 
         atomic_t                        owned_by_drv_count;
         struct list_head                done_list;
@@ -402,7 +408,7 @@ struct vb2_queue {
         unsigned int                    plane_sizes[VIDEO_MAX_PLANES];
 
         unsigned int                    streaming:1;
-        unsigned int                    retry_start_streaming:1;
+        unsigned int                    start_streaming_called:1;
 
         struct vb2_fileio_data          *fileio;