DRI gurus,
If I'm not mistaken, the current Linux graphics stack is as follows (excluding Wayland protocol and LLVM or GLAMOR-based approaches):
X11/OpenGL app -> libX/Mesa -> DDX driver/Mesa DRI module -> kernel DRM -> hardware
What's unclear to me is, in the case of an AGP graphics adapter, where does the AGP GART takes place in this stack (if applicable)?
Say I have an AGP ATI R300-based graphics adapter. In the above stack, DDX driver is x86-video-ati, Mesa DRI module is r300 (Classic or Gallium3D) and kernel DRM is radeon. (Am I still right?)
Obviously, this AGP graphics adapter nevertheless works flawlessly without AGP GART compiled in kernel or as module. This is at least true for the open source stack, I've tested it. Is my AGP graphics adapter thus running in what's known as PCI/PCIe mode? I've read all the AGP scatter/gather, texturing and fast writes things, but I can't see any difference performance-wise between having AGP GART compiled in kernel or as a module and no AGP GART. Is it because my usage doesn't stress the graphics subsystem enough or is it because PCI/PCIe mode is so amazing that AGP GART doesn't provide any performance enhancements? AGP GART however provides me nice stability issues ;-)
When compiled in kernel or as a module, is AGP GART only used for 3D hardware acceleration by the r300 Mesa DRI module (or is it by the radeon DRM? Or both?) or also by the xf86-video-ati DDX driver for XAA/EXA acceleration? And what about video acceleration?
What happens when the AGP GART isn't compiled in kernel or as a module? Is it simply a matter of skipping a participant (the AGP GART) in the graphics stack or are there different code paths in the DDX driver, Mesa DRI module and/or kernel DRM depending upon the availability of AGP GART or not?
Is the code path the same in the following situations: - no AGP GART at all; - AGP GART compiled in kernel or as a module but "options radeon agpmode=-1" set in /etc/modprobe.d/radeon-kms.conf.
Is setting a different AGP mode (1x, 2x, 4x, 8x) in /etc/modprobe.d/radeon-kms.conf only a hardware thing or are there different code paths taken in the various components of the graphics stack depending on the current AGP mode?
What happens if you compile AGP GART in kernel or as a module with a PCI/PCIe graphics adapter? Is it simply ignored? How? Out of Linux control at the hardware level or are there simply no code path taking advantages of the AGP GART in a PCI/PCIe graphics stack?
Finally is this assertion of the "radeon-KMS with AGP gfxcards" section of the radeonBuildHowTo [1] still true?
"AGP gfxcards have a lot of problems so if you have one it is good idea to test PCI/PCIE mode using radeon.agpmode=-1."
Thanks,
Émeric