Plugable

udlfb 0.4.0

Posted on 11. Nov, 2009 by Bernie Thompson in USB-VGA-165

[Update Dec 29, 2011: udlfb was promoted from the staging to the mainline kernel tree in 2.6.38. And in kernel 3.3 pagefault detection and console are enabled by default. See all our udlfb posts for the latest news.]

[Update March 14, 2010: udlfb versions have moved to being released with the Linux kernel. Update on udlfb support in Linux kernel 2.6.34]

[Update Feb 6, 2010: additional features and fixes post-0.4 are available at the plugable git page. Background is in later posts. One major udlfb patch with these changes has made it into linux-next (slated for 2.6.34), and additional patches will be coming as they're ready.]

This is a new release of the DisplayLink kernel framebuffer driver, udlfb.

udlfb was accepted into the Linux kernel staging tree of 2.6 a few months ago. It needs some work to add key features and get it moving from the staging tree, into mainline. Roberto De Iorio, the author of udlfb and displaylink-mod, is focusing on X server work, and is happy with this work happening in parallel to move udlfb forward.

This first release intentionally adds no fundamentally new features. It only gets udlfb up to sync with the displaylink-mod branch (up to Roberto’s last 0.3 release in July 2009) that has been in use the past few months. With this update, displaylink-mod users should be able to switch to this version of udlfb transparently.

Bug reports are very welcome, especially regressions or problems that would stand in the way of moving this driver forward out of staging (comments here are fine for bug reports).

New in 0.4.0 (since 0.2.3 currently in the Linux kernel staging tree)

• Add dynamic modeset support (from displaylink-mod 0.3 and libdlo)
• udlfb uses EDID to find the monitor’s preferred mode
• udlfb no longer has fixed mode tables – it’s able to set any mode (within the capabilities of the chip) dynamically, from the standard VESA timing characteristics of the mode
• Fix teardown synchronization issues (from displaylink-mod 0.3)
• Other minor changes related to probe/modeset (from displaylink-mod 0.3)
• Functionally identical to displaylink-mod 0.3
• Retains basic layout of udlfb to make diffs more transparent and understandable

Download

See the git project summary page at http://git.plugable.com/gitphp/index.php?p=udlfb&a=summary for information.

Switching from displaylink-mod to udlfb.

Both these drivers match against all displaylink devices. So you don’t want both loading on your system. To clear out displaylink-mod:

sudo rmmod displaylink
sudo rm /lib/modules/`uname -r`/extra/displaylink.ko
sudo depmod -a

Then download, compile, install udlfb 0.4

./configure
make
sudo make install
sudo depmod -a

Todo

• Merge in enhancements from Jaya Kumar’s displaylinkfb branch (defio support)
• Merge in enhancements from Bernie’s displaylink-mod branch (performance)
• Clear up remaining endian issues, to make sure it works on ARM and others
• Add performance metrics, and sysfs attributes to read/reset them
• Make allocation/use of backbuffer a runtime option, with param and sysfs switch
• Figure out what KMS (Kernel Mode Setting) means to framebuffer drivers
• Move from single URB with synchronous dispatch to ring of USB URBs, with asynchronous dispatch
• Enhance probe() to better handle chip type detection
• Enahance mode selection to better handle limits of DisplayLink chip
• Add simulated hardware cursor support, to prioritize mouse movement

Any feedback or ideas on these todos are very welcome. And, as always, patches are very welcome and will be incorporated as quickly as possible.

DisplayLink kernel framebuffer performance

Posted on 02. Nov, 2009 by Bernie Thompson in Programming

There are three codelines of Linux DisplayLink kernel frambuffer drivers currently in use:

• udlfb (Roberto De Ioris), which is in the Linux kernel staging tree of 2.6.31 and later, and is enabled by default in some recent distros (Ubuntu 9.10). Capable of working with all DisplayLink devices.
• displaylink-mod (Roberto De Ioris), which adds dynamic mode support and a few other minor changes
• displaylinkfb (Jaya Kumar), which tries out some innovative approaches (defio page-fault change detection), uses existing fbdev x servers without modification, but is much slower (80% slower on example test) than udlfb and displaylink-mod, which both use X damage information and RL/RAW compression
• udl (Marcus Glocker) for FreeBSD (not Linux) text/graphics console driver interface, which makes use of damage and ports Huffman-style compression from Florian Echtler’s libtubecable library. This support is currently ahead of what’s on Linux.

You can get more information about these drivers at http://libdlo.freedesktop.org/wiki/HowTo

In general, most of the demos and videos posted here use displaylink-mod. Some performance improvement patches are available at http://git.plugable.com/, but they are a relatively small improvement.

So any conclusions of of the performance work so far?

• Graphics benchmarks on Linux are in rough shape. Most practical approach so far has been simplistically using a combination of glxgears and a few select tests from x11perf. Good video playback tests needed.
• Jaya’s displaylinkfb driver tries some interesting concepts, but the lack of damage information from X means it runs much slower (up to 80% slower) than the alternatives for now.
• The original RL compression in udlfb by Henrik Bjerregaard Pedersen is surprisingly effective, even though it only uses one of RLE or RAW for each 255 pixel segment. It’s relatively CPU-efficient with simple inner loops, and decently USB-efficient in practice.
• The alternating RL/RAW algorithm in the master branch at http://git.plugable.com/ does slightly better, but it varies per test and is not dramatic.
• The shadow/back buffer that udlfb and displaylink-mod keep do provide a significant gain on maybe one out of every 4 tests or so, but X’s damage information is quite accurate — so saving that allocation and the ongoing reads/writes to that extra memory by forgoing the back buffer is definitely viable and should at least be a module option. The ‘noback’ branch at http://git.plugable.com/ has this removed (despite what one comment checkin says), for anyone who wants to try it.

Perhaps the best and quickest path forward to getting support more widely distributed is

• Bringing udlfb up to snuff, since there are just a few functional changes in the displaylink-mod branch, and udlfb already is going through the staging->mainline confidence building process
• For the matching X server, it would be great to have kernel driver that works with both the standard http://cgit.freedesktop.org/xorg/driver/xf86-video-fbdev/ at some level of performance, or with the custom displaylink xserver at some (better) level of performance. Then move things (like damage support, which is key to performance) from the displaylink server to the fbdev server in a standardized way over time.
• Then there’s a bunch of other more involved work to come on configuration and coexistence with multiple graphics controllers. Rough plans are visible in the fog here, but they involve other projects and people.

Plug and Play Multiseat Talk Given at Linux Plumbers Conference 2009

Posted on 27. Oct, 2009 by Bernie Thompson in UD-160-A

The video of the talk mentioned in this previous post, is now online (recently moved from the Linux Foundation site, to youtube in 4 parts).

Covering:

• The potential of USB multiseat
• Some background and earlier work
• Live demo of USB multiseat with three Plugable devices
• Walkthrough of the multiseat scripts for Ubuntu 9.04 (available here)

The same device used in the demos is available for purchase here: