DisplayLink Linux Rotation

[updated 8/14/2011 with latest udlfb info]

The xorg generic fbdev driver (xf86-video-fbdev) supports rotation with a custom option (note it disables DGA and xrandr when it rotated mode). DisplayLink devices can use this driver two ways:

  • With the udlfb fb_defio option enabled to detect framebuffer writes based on page faults
  • With the fbdev xorg server patched to report damage (writes to the framebuffer). Instructions for compiling and adding the ReportDamage option to xorg.conf are at the end of this post.

To enable rotation, in the xorg.conf “Device” section, add a Rotate option, setting it to CW, CCW, or UD.

Section "Device"
  Identifier "dl"
  Driver "fbdev"
  Option "rotate" "CCW"
  Option "ReportDamage" "true"
  Option "fbdev" "/dev/fb1"

One downside is rendering is significantly slower, as the extra work of rotation is done at the X level and the page-fault behavior of defio means even small updates refresh much of the screen. Here’s some numbers that can be compared to earlier posts (in non-rotate mode):

bernie@bernie-aspireone:~/git/misc-udlfb$ ./udlfb-perf.sh fb0 gtkperf -a
Xlib:  extension "RANDR" missing on display ":3.0".
GtkPerf 0.40 - Starting testing: Fri Jan  1 09:47:20 2010

GtkEntry - time:  0.00
GtkComboBox - time:  3.73
GtkComboBoxEntry - time:  2.08
GtkSpinButton - time:  0.49
GtkProgressBar - time:  0.69
GtkToggleButton - time:  0.47
GtkCheckButton - time:  0.44
GtkRadioButton - time:  0.79
GtkTextView - Add text - time:  2.30
GtkTextView - Scroll - time:  1.24
GtkDrawingArea - Lines - time:  4.14
GtkDrawingArea - Circles - time:  4.56
GtkDrawingArea - Text - time:  4.11
GtkDrawingArea - Pixbufs - time:  0.90
Total time: 25.96


model name      : Intel(R) Atom(TM) CPU N270   @ 1.60GHz
model name      : Intel(R) Atom(TM) CPU N270   @ 1.60GHz
cpu MHz         : 800.000
cpu MHz         : 800.000
MemTotal:        2052144 kB
Framebuffer Mode: 1920,1080

Rendered bytes:  1238913024 (total pixels * Bpp)
Identical bytes: 1070725304 (skipped via shadow buffer check)
sent bytes:      125732225 (compressed usb data, including overhead)
K CPU cycles:    7478653 (transpired, may include context switches)

% pixels found to be unchanged: 86.00 %
Compression of changed pixels : 25.00 %
Total CPU cycles spent per input pixel: 6
Total CPU cycles spent per output pixel: 59
USB Mbps: 35.52 (theoretical USB 2.0 peak 480)

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