[PATCH weston 0/4] Optimize pixman renderer

[Fabien Lahoudere]

Optimizes pixman renderer by:
- optimizing compositing damage in DRM/pixman

  Reduce Weston's CPU usage by avoiding unnecessary compositing when updating 
  the shadow buffer in pixman-renderer, under the DRM backend. The test was a 
  proprietary graphical X11 application in demo mode. The effect is a drop of
  total system CPU usage from 0.41 to 0.33.

- optimizing shadow buffer usage

  The shadow framebuffer is an intermediate buffer where the scene is composited
  and then copied from the shadow to the actual hardware buffer. This extra step
  costs memory bandwidth compared to compositing directly into a hardware buffer.

  Weston's DRM-backend with the Pixman-renderer uses a shadow framebuffer by
  default. Especially on systems with dedicated VRAM, read-modify-write cycles
  (a.k.a blending) into the scanout-capable buffer can be very slow. Also the
  scanout pixel format may not be optimal for compositing. Therefore Weston takes
  the safe default to always use a shadow framebuffer.

  However, in our use case, the hardware does not have dedicated VRAM behind a
  relatively slow bus, and the graphical load has practically no blending. We 
  can reduce Weston's CPU usage quite a lot by not using the shadow frambuffer.
  
  We test on proprietary graphical X11 application in demo mode. Using perf we
  measure that this change improve weston CPU usage by 13% with dual display and
  11% with clone mode.

Pekka Paalanen (4):
  pixman,drm: do not composite previous damage
  pixman: make shadow buffer optional
  compositor-drm: expose global shadow flag for pixman
  main: add setting for DRM/pixman shadow framebuffer

 compositor/main.c               |  3 ++
 libweston/compositor-drm.c      | 30 +++++++------
 libweston/compositor-drm.h      |  3 ++
 libweston/compositor-fbdev.c    |  3 +-
 libweston/compositor-headless.c |  3 +-
 libweston/compositor-rdp.c      |  5 ++-
 libweston/compositor-wayland.c  |  3 +-
 libweston/compositor-x11.c      |  6 ++-
 libweston/pixman-renderer.c     | 99 ++++++++++++++++++++++++++++-------------
 libweston/pixman-renderer.h     | 13 +++++-
 man/weston-drm.man              |  4 ++
 11 files changed, 118 insertions(+), 54 deletions(-)

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1.8.3.1