We've talked all too often about AtomBIOS and there being two different open-source drivers that support the same ATI Radeon hardware with the key architectural difference between the two just being the use of this video BIOS abstraction layer. From the beginning, AMD was planning to have their Novell partners use AtomBIOS when writing this new (at the time, R500/600) driver, but the developers ultimately declined. These developers have expressed their opinions on AtomBIOS, which range from it being an unbearable mess to this design being nothing more than writing open-source code to power someone else's closed-source work. However, under pressure by AMD, the developers are now preparing to use AtomBIOS to a much greater extent within the xf86-video-radeonhd driver. In this article we'll tell you more about what's gone on and where you can checkout this AtomBIOS-bearing RadeonHD driver.
In March we had explained the Radeon vs. RadeonHD driver situation in full. In theory, this common API for programming all of ATI's R400+ graphics hardware is a godsend, but in the real world it's the RadeonHD developers that argue its benefit and expose its faults -- going as far as calling it legacy code and untouchable byte-code. AMD's Alex Deucher and Red Hat's David Airlie use AtomBIOS within the xf86-video-ati driver so that they can quickly deploy the basic mode-setting support and other fundamental work, thereby allowing them more time to focus on the driver's "bells and whistles" such as OpenGL hardware acceleration and other features we've previously shared (Open-Source ATI Driver Achieves 3D Success). Meanwhile, the RadeonHD developers prefer hard-coding the driver and accessing all registers directly. These differences plus other factors have led to little cooperation between the xf86-video-radeonhd and xf86-video-ati driver development groups until recently when the xf86-video-ati 2D and DRM work started being adopted by xf86-video-radeonhd.
One very recent example that highlights the difference between these two programming models is with the recently introduced Radeon HD 4850 and Radeon HD 4870. AMD had delivered same-day support with the fglrx driver (their proprietary driver also uses AtomBIOS), but on the open-source side there was even same-day Linux support using xf86-video-ati. While there are plenty of architectural changes between the R600 series and the RV770, the AtomBIOS on the graphics card's ROM is able to conceal most of these differences to the driver.
With writing a simple patch that just adds in the RV770 chip family identification and PCI IDs for our graphics cards we were able to experience open-source Radeon HD 4850 and open-source Radeon HD 4870 support. We had experienced a DDC (Display Data Channel) bug that caused the incorrect resolution to be set by default, but that's only a minor issue for now. With the current xf86-video-radeonhd driver, a trivial patch cannot be created to add the RV770 graphics processor support. Developers are left to either utilize AtomBIOS or wait for AMD to release documentation -- whether it be publicly or under NDA to Novell -- in order to hard-code the support for this high-end graphics processor. As we have seen from past product launches within a same product generation, it has taken the RadeonHD driver weeks to deliver even proper mode-setting support.
However, we can confirm that the RadeonHD driver now is moving to an AtomBIOS model. In a branch of xf86-video-radeonhd driver, they have now created an atombios_support branch that contains this initial work. However, before going to fire up git, be forewarned that this atombios_support branch is still considered experimental and isn't yet complete. We've been experimenting with this branch for the past 24 hours with the Radeon HD 4850/4870 as well as earlier R500 and R600 graphics cards, but there are some areas of the AtomBIOS support that aren't yet addressed that will likely lead to show-stopping issues for the time being. However, this AtomBIOS support is rapidly developing and we would expect many of the issues to be worked out within a matter of days. Following that, this AtomBIOS support will likely be merged to the master branch. Previously AtomBIOS was used within the RadeonHD driver for just supporting parts of the graphics processor they didn't have specifications on at the time of commencing development work (such as with the PCIEPHY transmitter).
Once the AtomBIOS support merges to the master branch of the xf86-video-radeonhd driver, the differences between that and xf86-video-ati will be even less. Ideally, this will lead to greater cooperation between the two groups of developers and settling for a de facto standard driver for the R600+ GPUs (likely xf86-video-radeonhd). Once the AtomBIOS support has been merged to master -- as well as their "quick and dirty" 2D branch -- they will begin focusing on open-source 3D support for the Radeon HD 4800 series. We'll have more on these open-source developments in the coming days and weeks.