[Resend][RFC PATCH v6 0/2] Introduce buffer synchronization framework

Inki Dae inki.dae at samsung.com
Tue Aug 13 08:56:27 EDT 2013

Just adding more detailed descriptions.

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
   may be final RFC.

   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.

   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 similar procedure. Web browser and its web
   app are different processs. Web app draws something in its own buffer,
   and then the web browser gets a window buffer from Xorg, and then composes
   those two buffers using GPU.

   Thus, in such cases, a shared buffer could be broken when one process draws
   something in a shared buffer using CPU while Composite manager is composing
   two buffers - X client's front buffer and Composite manger's back buffer, or
   web app's front buffer and web browser's back 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):
  [RFC PATCH v6] dmabuf-sync: Add a buffer synchronization framework
  [RFC PATCH v2] dma-buf: Add user interfaces for dmabuf sync support.

 Documentation/dma-buf-sync.txt |  285 +++++++++++++++++
 drivers/base/Kconfig           |    7 +
 drivers/base/Makefile          |    1 +
 drivers/base/dma-buf.c         |   85 +++++
 drivers/base/dmabuf-sync.c     |  678 ++++++++++++++++++++++++++++++++++++++++
 include/linux/dma-buf.h        |   16 +
 include/linux/dmabuf-sync.h    |  191 +++++++++++
 7 files changed, 1263 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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