Chris Wilson e58f637bb9 drm/kms: Add a module parameter to disable polling
Polling for a VGA device on an old system can be quite expensive,
causing latencies on the order of 600ms. As we hold the mode mutex for
this time and also need the same mutex to move the cursor, we trigger a
user-visible stall.

The real solution would involve improving the granulatity of the
locking and so perhaps performing some of the probing not under the lock
or some other updates can be done under different locks. Also reducing the
cost of probing for a non-existent monitor would be worthwhile. However,
exposing a parameter to disable polling is a simple workaround in the
meantime.

In order to accommodate users turning polling on and off at runtime, the
polling is potentially re-enabled on every probe. This is coupled to
the user calling xrandr, which seems to be a vaild time to reset the
polling timeout since the information on the connection has just been
updated. (The presumption being that all connections are probed in a
single xrandr pass, which is currently valid.)

References:

  Bug 29536 - 2.6.35 causes ~600ms latency every 10s
  https://bugs.freedesktop.org/show_bug.cgi?id=29536

  Bug 16265 - Why is kslowd accumulating so much CPU time?
  https://bugzilla.kernel.org/show_bug.cgi?id=16265

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
2010-09-07 08:04:11 +10:00
..
2010-08-04 09:46:06 +10:00
2010-05-18 15:57:05 +10:00
2010-08-10 10:47:00 +10:00
2010-05-18 15:57:05 +10:00
2010-08-10 08:20:20 +10:00
2010-08-20 08:48:34 +10:00
2010-08-04 09:46:06 +10:00

************************************************************
* 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