README.Porting
author Sam Lantinga <slouken@libsdl.org>
Fri, 29 Feb 2008 13:55:44 +0000
branchSDL-1.2
changeset 4139 568c9b3c0167
parent 1484 b2b476a4a73c
child 1946 103760c3a5dc
permissions -rw-r--r--
* Added configure option --enable-screensaver, to allow enabling the screensaver by default.
* Use XResetScreenSaver() instead of disabling screensaver entirely.

Full discussion summary from Erik on the SDL mailing list:

Current behaviour
=================

SDL changes the user's display power management settings without
permission from the user and without telling the user.

The interface that it uses to do so is DPMSDisable/DPMSEnable, which
should only ever be used by configuration utilities like KControl, never
by normal application programs, let alone by the libraries that they
use. Using an interface that is not at all intended for what SDL tries
to achieve means that it will not work as it should. Firstly, the power
management is completely disabled during the whole lifetime of the SDL
program, not only when it should be. Secondly, it makes SDL
non-reentrant, meaning that things will break when multiple SDL programs
are clients of the same X server simultaneously. Thirdly, no cleanup
mechanism ensures that the setting is restored if the client does not do
that (for example if it crashes).

In addition to that, this interface is broken on xorg,
[http://bugs.freedesktop.org/show_bug.cgi?id=13962], so what SDL tries
to do does not work at all on that implementation of the X Window
System. (The reason that the DPMSEnable works in KControl is that it
calls DPMSSetTimeout immediately after,
[http://websvn.kde.org/tags/KDE/3.5.9/kdebase/kcontrol/energy/energy.cpp?annotate=774532#l343]).


The problems that the current behaviour causes
==============================================
1. Information leak. When the user is away, someone might see what the
user has on the display when the user counts on the screensaver
preventing this. This does not even require physical access to the
workstation, it is enough to see it from a distance.
2. Draining battery. An SDL program that runs on a laptop will quickly
drain the battery while the user is away. The system will soon shut down
and require recharging before being usable again, while it should in
fact have consumed very little energy if the user's settings would have
been obeyed.
3. Wasting energy. Even if battery issues are not considered, energy as
such is wasted.
4. Display wear. The display may be worn out.


The problems that the current behaviour tries to solve
======================================================

1. Preventing screensaver while playing movies.
Many SDL applications are media players. They have reasons to prevent
screensavers from being activated while a movie is being played. When a
user clicks on the play button it can be interpreted as saying "play
this movie, but do not turn off the display while playing it, because I
will watch it even though I do not interact with the system".

2. Preventing screensaver when some input bypasses X.
Sometimes SDL uses input from another source than the X server, so
that the X server is bypassed. This obviously breaks the screensaver
handling. SDL tries to work around that.

3. Preventing screensaver when all input bypasses X.
There is something called Direct Graphics Access mode, where a
program takes control of both the display and the input devices from the
X server. This obviously means that the X server can not handle the
screensaver alone, since screensaver handling depends on input handling.
SDL does not do what it should to help the X server to handle the
screensaver. Nor does SDL take care of screeensaver handling itself. SDL
simply disables the screensaver completely.


How the problems should be solved
=================================

The correct way for an application program to prevent the screensaver
under X is to call XResetScreenSaver. This was recently discovered and
implemented by the mplayer developers,
[http://svn.mplayerhq.hu/mplayer?view=rev&revision=25637]. SDL needs to
wrap this in an API call (SDL_ResetScreenSaver) and implement it for the
other video targets (if they do not have a corresponding call, SDL
should do what it takes on that particular target, for example sending
fake key events).

1. When a movie is played, the player should reset the screensaver when
the animation is advanced to a new frame. The same applies to anything
similar, like slideshows.

2. When the X server is handling input, it must handle all input
(keyboards, mice, gamepads, ...). This is necessary, not only to be able
to handle the screensaver, but also so that it can send the events to
the correct (the currently active) client. If there is an input device
that the X server can not handle for some reason (such as lack of Plug
and Play capability), the program that handles the device as a
workaround must simulate what would happen if the X server would have
handled the device, by calling XResetScreenSaver when input is received
from the device.

3. When the X server is not handling the input, it depends on the
program that does to call XResetScreenSaver whenever an input event
occurs. Alternatively the program must handle the screensaver countdown
internally and call XActivateScreenSaver.
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* Porting To A New Platform
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  The first thing you have to do when porting to a new platform, is look at
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include/SDL_platform.h and create an entry there for your operating system.
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The standard format is __PLATFORM__, where PLATFORM is the name of the OS.
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Ideally SDL_platform.h will be able to auto-detect the system it's building
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on based on C preprocessor symbols.
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There are two basic ways of building SDL at the moment:
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1. The "UNIX" way:  ./configure; make; make install
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   If you have a GNUish system, then you might try this.  Edit configure.in,
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   take a look at the large section labelled:
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	"Set up the configuration based on the target platform!"
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   Add a section for your platform, and then re-run autogen.sh and build!
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2. Using an IDE:
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   If you're using an IDE or other non-configure build system, you'll probably
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   want to create a custom SDL_config.h for your platform.  Edit SDL_config.h,
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   add a section for your platform, and create a custom SDL_config_{platform}.h,
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   based on SDL_config.h.minimal and SDL_config.h.in
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   Add the top level include directory to the header search path, and then add
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   the following sources to the project:
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	src/*.c
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	src/audio/*.c
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	src/cdrom/*.c
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	src/cpuinfo/*.c
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	src/events/*.c
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	src/file/*.c
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	src/joystick/*.c
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	src/stdlib/*.c
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	src/thread/*.c
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	src/timer/*.c
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	src/video/*.c
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	src/audio/disk/*.c
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	src/video/dummy/*.c
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	src/joystick/dummy/*.c
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	src/cdrom/dummy/*.c
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	src/thread/generic/*.c
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	src/timer/dummy/*.c
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	src/loadso/dummy/*.c
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Once you have a working library without any drivers, you can go back to each
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of the major subsystems and start implementing drivers for your platform.
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If you have any questions, don't hesitate to ask on the SDL mailing list:
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	http://www.libsdl.org/mailing-list.php
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Enjoy!
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	Sam Lantinga				(slouken@libsdl.org)
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