doc/driver-model/UDM-video.txt

The U-Boot Driver Model Project
===============================
Video output analysis
=====================
Marek Vasut <marek.vasut@gmail.com>
2012-02-20

I) Overview
-----------

The video drivers are most often registered with video subsystem. This subsystem
often expects to be allowed access to framebuffer of certain parameters. This
subsystem also provides calls for STDIO subsystem to allow it to output
characters on the screen. For this part, see [ UDM-stdio.txt ].

Therefore the API has two parts, the video driver part and the part where the
video driver core registers with STDIO API.

The video driver part will follow the current cfb_console approach, though
allowing it to be more dynamic.

II) Approach
------------

Registering the video driver into the video driver core is done by calling the
following function from the driver probe() function:

  video_device_register(struct instance *i, GraphicDevice *gd);

Because the video driver core is in charge or rendering characters as well as
bitmaps on the screen, it will in turn call stdio_device_register(i, so), where
"i" is the same instance as the video driver's one. But "so" will be special
static struct stdio_device_ops handling the character output.


III) Analysis of in-tree drivers
--------------------------------

  arch/powerpc/cpu/mpc8xx/video.c
  -------------------------------
  This driver copies the cfb_console [ see drivers/video/cfb_console.c ]
  approach and acts only as a STDIO device. Therefore there are currently two
  possible approaches, first being the conversion of this driver to usual STDIO
  device and second, long-term one, being conversion of this driver to video
  driver that provides console.

  arch/x86/lib/video.c
  --------------------
  This driver registers two separate STDIO devices and should be therefore
  converted as such.

  board/bf527-ezkit/video.c
  -------------------------
  This driver seems bogus as it behaves as STDIO device, but provides no input
  or output capabilities. It relies on DEV_EXT_VIDEO, which is no longer in use
  or present otherwise than as a dead code/define.

  board/bf533-stamp/video.c
  -------------------------
  This driver seems bogus as it behaves as STDIO device, but provides no input
  or output capabilities. It relies on DEV_EXT_VIDEO, which is no longer in use
  or present otherwise than as a dead code/define.

  board/bf548-ezkit/video.c
  -------------------------
  This driver seems bogus as it behaves as STDIO device, but provides no input
  or output capabilities. It relies on DEV_EXT_VIDEO, which is no longer in use
  or present otherwise than as a dead code/define.

  board/cm-bf548/video.c
  ----------------------
  This driver seems bogus as it behaves as STDIO device, but provides no input
  or output capabilities. It relies on DEV_EXT_VIDEO, which is no longer in use
  or present otherwise than as a dead code/define.