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Running USB monitors

The kernel has also gained a simple DRM/KMS driver by the name of udl. It implements the DisplayLink protocol, which is used for controlling USB monitors. The driver was written by Dave Airlie and supports basic UDL hardware functionality. The recently unveiled X Server driver xf86-video-modesetting can be used with the udl KMS-driver; this DDX driver relies entirely on KMS and, in contrast to the Intel, nouveau and Radeon X.Org drivers (which also make extensive use of KMS), includes no graphics-chip-specific functionality. The mode setting driver is therefore able to work in conjunction with other KMS drivers including, for example, the driver for the GMA500 graphics core in Intel's US15W chipset (Poulsbo).

Large parts of the udl code are based on udlfb, a framebuffer USB DisplayLink driver which has been part of the kernel for some time and offers similar functionality. By porting it to the DRM/KMS infrastructure used by other graphics drivers, it is able to benefit from the shared foundation. Some, for example, are important for operating Wayland, a graphics system intended as an alternative to X Server. Udl also supports hot plugging.

Switched on

The udl driver is just one of many changes to the kernel, X Server, etc. currently under development designed to enable graphics hardware to be connected and disconnected from live systems without requiring the user to restart X Server or undertake any other intervention.

One further component is a basic framework for prime/dma-buf, which enables graphics cores to swap data with minimal overhead and which has also been merged into 3.4. In the long term, this is intended to improve support for hot pluggable graphics chips, by allowing, say, an NVIDIA chip to pass images it has calculated to the processor graphics core for output without too much effort. GeForce chips in the current Optimus laptops, for example, output images in such a way, as does AMD's latest hybrid graphics solution. Although no drivers are using the new infrastructure as yet, relevant extensions to the nouveau, i915, udl, exynos and omap DRM/KMS drivers are expected to be merged into Linux 3.5.


A platform driver for the gmux hardware used by some Apple systems to switch between two graphics chips has been added to the Linux kernel. The driver does not add full support for switching the second graphics chip on and off, but merely allows the screen brightness to be regulated on supported Apple systems.

New functions enable the information required to correctly address monitors (EDID/extended display identification data) to be passed to the kernel during booting or to be loaded later via sysfs. This can be a useful workaround when KVM switches corrupt the information or when monitors and graphics drivers are unable to supply the correct EDID information. Details can be found in this OSADL (Open Source Automation Development Lab) article. For simple cases, the kernel parameter "video", which has long been supported by the kernel, is sufficient for specifying the mode required. "video=1024x768-24@75", for example, specifies a resolution of 1024×768 pixels with 24 bit colour and a screen refresh rate of 75 hertz.

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