Radeon Kernel Driver Deprecates UMS Mode-Setting

The open-source AMD Radeon Linux graphics stack has been deprecating the user-space mode-setting (UMS) code for a while and is now finally making the kernel-space mode-setting (KMS) support the default Radeon interface for the Linux kernel.

This news isn't a surprise that the final blows are being delivered to Radeon UMS support. Years ago Intel quickly killed off their UMS support with the merging of KMS support. Since 2012, Nouveau also killed off non-KMS support with their open-source NVIDIA driver. AMD has been slowly moving in this direction of abandoning user-space mode-setting support with their KMS driver being stable, early shortcomings having been addressed, and KMS drivers being the certain future on Linux.

For recent generations of AMD/ATI GPUs, they have only been enabling new product support along the KMS code-paths, but UMS has still been supported for existing hardware. Going back to last June, plans were laid to drop Radeon UMS support and the xf86-video-ati 6.xx.xx DDX driver was the last release to support the user-space mode-setting.

With the release of xf86-video-ati 7.0.0 in November, the X.Org driver for Radeon GPUs now only handles kernel mode-setting. While for a couple years most desktop Linux distributions have been using the KMS graphics drivers, maintaining user-space mode-setting after the introduction of Radeon KMS support was mostly done for continuing to properly support the GPUs in the older enterprise Linux distributions running on older kernel releases. Fortunately, modern enterprise Linux distributions are in a KMS-enabled world.

AMD's Christian König sent out a patch on this Sunday morning to deprecate UMS support within the DRM Radeon kernel driver. Per the mailing list message:

KMS support is out and stable for a couple of years now and the userspace code has deprecated or abandoned the old UMS interface.

So make the KMS interface the default and deprecate the UMS interface in the kernel as well.

This change will likely come for the Linux 3.9 kernel and the just-added DRM_RADEON_UMS kernel configuration option should only be enabled if for some reason you plan to stick in an old xf86-video-ati driver on your Linux system.

While this is just about a change specific to the Linux kernel driver, the only major impact that this continued migration to a KMS-only world has on users is for those on BSD, Solaris, and other Unix-like file-systems. While Oracle Solaris and FreeBSD in their latest releases have ported an Intel KMS driver, these operating systems still have graphics drivers in a poor state with no mainline Radeon or Nouveau driver for supporting the latest graphics cards from AMD and NVIDIA. Some of these operating systems are sticking to ancient UMS drivers.