[PATCH v7 0/2] Introduce buffer synchronization framework

Inki Dae inki.dae at samsung.com
Wed Aug 21 06:33:33 EDT 2013

Hi all,

This patch set introduces a buffer synchronization framework based
on DMA BUF[1] and based on ww-mutexes[2] for lock mechanism, and
has been rebased on linux-3.11-rc6.

The purpose of this framework is to provide not only buffer access
control to CPU and CPU, and CPU and DMA, and DMA and DMA but also
easy-to-use interfaces for device drivers and user application.
In addtion, this patch set suggests a way for enhancing performance.

Changelog v7:
Fix things pointed out by Konrad Rzeszutek Wilk,
- Make sure to unlock and unreference all dmabuf objects
  when dmabuf_sync_fini() is called.
- Add more comments.
- Code cleanups.

Changelog v6:
- Fix sync lock to multiple reads.
- Add select system call support.
  . Wake up poll_wait when a dmabuf is unlocked.
- Remove unnecessary the use of mutex lock.
- Add private backend ops callbacks.
  . This ops has one callback for device drivers to clean up their
    sync object resource when the sync object is freed. For this,
    device drivers should implement the free callback properly.
- Update document file.

Changelog v5:
- Rmove a dependence on reservation_object: the reservation_object is used
  to hook up to ttm and dma-buf for easy sharing of reservations across
  devices. However, the dmabuf sync can be used for all dma devices; v4l2
  and drm based drivers, so doesn't need the reservation_object anymore.
  With regared to this, it adds 'void *sync' to dma_buf structure.
- All patches are rebased on mainline, Linux v3.10.

Changelog v4:
- Add user side interface for buffer synchronization mechanism and update
  descriptions related to the user side interface.

Changelog v3:
- remove cache operation relevant codes and update document file.

Changelog v2:
- use atomic_add_unless to avoid potential bug.
- add a macro for checking valid access type.
- code clean.

For generic user mode interface, we have used fcntl and select system
call[3]. As you know, user application sees a buffer object as a dma-buf
file descriptor. So fcntl() call with the file descriptor means to lock
some buffer region being managed by the dma-buf object. And select() call
means to wait for the completion of CPU or DMA access to the dma-buf
without locking. For more detail, you can refer to the dma-buf-sync.txt
in Documentation/

There are some cases we should use this buffer synchronization framework.
One of which is to primarily enhance GPU rendering performance on Tizen
platform in case of 3d app with compositing mode that 3d app draws
something in off-screen buffer, and Web app.

In case of 3d app with compositing mode which is not a full screen mode,
the app calls glFlush to submit 3d commands to GPU driver instead of
glFinish for more performance. The reason we call glFlush is that glFinish
blocks caller's task until the execution of the 2d commands is completed.
Thus, that makes GPU and CPU more idle. As result, 3d rendering performance
with glFinish is quite lower than glFlush. However, the use of glFlush has
one issue that the a buffer shared with GPU could be broken when CPU
accesses the buffer at once after glFlush because CPU cannot be aware of
the completion of GPU access to the buffer. Of course, the app can be aware
of that time using eglWaitGL but this function is valid only in case of the
same process.

The below summarizes how app's window is displayed on Tizen platform:
1. X client requests a window buffer to Xorg.
2. X client draws something in the window buffer using CPU.
3. X client requests SWAP to Xorg.
4. Xorg notifies a damage event to Composite Manager.
5. Composite Manager gets the window buffer (front buffer) through
6. Composite Manager composes the window buffer and its own back buffer
   using GPU. At this time, eglSwapBuffers is called: internally, 3d
   commands are flushed to gpu driver.
7. Composite Manager requests SWAP to Xorg.
8. Xorg performs drm page flip. At this time, the window buffer is
   displayed on screen.

Web app based on HTML5 also has the same issue. Web browser and its web app
are different process. The web app draws something in its own pixmap buffer,
and then the web browser gets a window buffer from Xorg, and then composites
the pixmap buffer with the window buffer. And finally, page flip.

Thus, in such cases, a shared buffer could be broken as one process draws
something in pixmap buffer using CPU, when other process composites the
pixmap buffer with window buffer using GPU without any locking mechanism.
That is why we need user land locking interface, fcntl system call.

And last one is a deferred page flip issue. This issue is that a window
buffer rendered can be displayed on screen in about 32ms in worst case:
assume that the gpu rendering is completed within 16ms.
That can be incurred when compositing a pixmap buffer with a window buffer
using GPU and when vsync is just started. At this time, Xorg waits for
a vblank event to get a window buffer so 3d rendering will be delayed
up to about 16ms. As a result, the window buffer would be displayed in
about two vsyncs (about 32ms) and in turn, that would show slow

For this, we could enhance the responsiveness with locking
mechanism: skipping one vblank wait. I guess in the similar reason,
Android, Chrome OS, and other platforms are using their own locking
mechanisms; Android sync driver, KDS, and DMA fence.

The below shows the deferred page flip issue in worst case,

               |------------ <- vsync signal
               |<------ DRI2GetBuffers
               |------------ <- vsync signal
               |<------ Request gpu rendering
          time |
               |<------ Request page flip (deferred)
               |------------ <- vsync signal
               |<------ Displayed on screen
               |------------ <- vsync signal

Inki Dae

[1] http://lwn.net/Articles/470339/
[2] https://patchwork.kernel.org/patch/2625361/
[3] http://linux.die.net/man/2/fcntl

Inki Dae (2):
  dmabuf-sync: Add a buffer synchronization framework
  dma-buf: Add user interfaces for dmabuf sync support

 Documentation/dma-buf-sync.txt |  286 ++++++++++++++++
 drivers/base/Kconfig           |    7 +
 drivers/base/Makefile          |    1 +
 drivers/base/dma-buf.c         |   85 +++++
 drivers/base/dmabuf-sync.c     |  706 ++++++++++++++++++++++++++++++++++++++++
 include/linux/dma-buf.h        |   16 +
 include/linux/dmabuf-sync.h    |  236 ++++++++++++++
 7 files changed, 1337 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/dma-buf-sync.txt
 create mode 100644 drivers/base/dmabuf-sync.c
 create mode 100644 include/linux/dmabuf-sync.h


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