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
DRI2GetBuffers.
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
responsiveness.
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
Thanks,
Inki Dae
References:
[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
--
1.7.5.4
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