Sometimes we want to disable all the screens on a system, because that
will allow the graphics card to be put into low-power states. The
problem is that, for example, while all screens are disabled, if we
get a hotplug interrupt, fbcon will decide to set a mode instead of
keeping everything disabled, which will remove us from our low power
states.
Let's assume that if there's a DRM master, it will be able to do
whatever is appropriate when we get the hotplug.
This problem can be reproduced by the runtime PM test program from
intel-gpu-tools: we disable all the screens so the graphics device can
be put into D3, then something triggers a hotplug interrupt, fbcon
sets a mode and breaks our test suite. The problem can be reproduced
more easily by the "i2c" subtest.
Other approaches considered for the problem:
- Return "false" if "bound == 0" and the caller of
drm_fb_helper_is_bound is a hotplug handler. This would break
the case where the machine boots with no outputs connected, then
the user plugs a monitor.
- Add a new IOCTL to force fbcon to not set modes. This would keep
all the current applications behaving the same, but adding a new
IOCTL is not always the greatest idea.
- Return false only if "dev->primary->master && bound == 0". This
was my first implementation, but Chris suggested we should do
the check irrespective of the "bound" variable.
Thanks to Daniel Vetter for the investigation, ideas and the
implementation of the hotplug alternative.
v2: - Do the check first, irrespective of "bound".
- Cc dri-devel
Cc: dri-devel@lists.freedesktop.org
Credits-to: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
************************************************************
* For the very latest on DRI development, please see: *
* http://dri.freedesktop.org/ *
************************************************************
The Direct Rendering Manager (drm) is a device-independent kernel-level
device driver that provides support for the XFree86 Direct Rendering
Infrastructure (DRI).
The DRM supports the Direct Rendering Infrastructure (DRI) in four major
ways:
1. The DRM provides synchronized access to the graphics hardware via
the use of an optimized two-tiered lock.
2. The DRM enforces the DRI security policy for access to the graphics
hardware by only allowing authenticated X11 clients access to
restricted regions of memory.
3. The DRM provides a generic DMA engine, complete with multiple
queues and the ability to detect the need for an OpenGL context
switch.
4. The DRM is extensible via the use of small device-specific modules
that rely extensively on the API exported by the DRM module.
Documentation on the DRI is available from:
http://dri.freedesktop.org/wiki/Documentation
http://sourceforge.net/project/showfiles.php?group_id=387
http://dri.sourceforge.net/doc/
For specific information about kernel-level support, see:
The Direct Rendering Manager, Kernel Support for the Direct Rendering
Infrastructure
http://dri.sourceforge.net/doc/drm_low_level.html
Hardware Locking for the Direct Rendering Infrastructure
http://dri.sourceforge.net/doc/hardware_locking_low_level.html
A Security Analysis of the Direct Rendering Infrastructure
http://dri.sourceforge.net/doc/security_low_level.html
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