On 2020-05-22 12:40 p.m., Christian König wrote: > Am 20.05.20 um 18:25 schrieb Michel Dänzer: >> On 2020-05-20 4:43 p.m., Christian König wrote: >>> Am 13.05.20 um 13:03 schrieb Christian König: >>>> Unfortunately AGP is still to widely used as we could just drop >>>> support for using its GART. >>>> >>>> Not using the AGP GART also doesn't mean a loss in functionality since >>>> drivers will just fallback to the driver specific PCI GART. >>>> >>>> For now just deprecate the code and don't enable the AGP GART in TTM >>>> even when general AGP support is available. >>> So I've used an ancient system (32bit) to setup a test box for this. >>> >>> >>> The first GPU I could test is an RV280 (Radeon 9200 PRO) which is easily >>> 15 years old. >>> >>> What happens in AGP mode is that glxgears shows artifacts during >>> rendering on this system. >>> >>> In PCI mode those rendering artifacts are gone and glxgears seems to >>> draw everything correctly now. >>> >>> Performance is obviously not comparable, cause in AGP we don't render >>> all triangles correctly. >>> >>> >>> The second GPU I could test is an RV630 PRO (Radeon HD 2600 PRO AGP) >>> which is more than 10 years old. >>> >>> As far as I can tell this one works in both AGP and PCIe mode perfectly >>> fine. >>> >>> Since this is only a 32bit system I couldn't really test any OpenGL game >>> that well. >>> >>> But for glxgears switching from AGP to PCIe mode seems to result in a >>> roughly 5% performance drop. >>> >>> The surprising reason for this is not the better TLB performance, but >>> the lack of USWC support for the PCIe GART in radeon. >> I suspect the main reason it's only 5% is that PCIe GART page tables are >> stored in VRAM, so they don't need to be fetched across the PCIe link >> (and presumably it has more than one TLB entry as well). The difference >> is much bigger with native AGP ASICs with PCI GART. > > Do you have some hardware you could give that a try on? As I mentioned before, I tested this many times on my AGP PowerBooks back in the day. The result was always a similar, big hit with PCI GART vs AGP (even just 1x). I haven't seen any reason to believe this has changed. > While I agree that it means a performance regression, this is a rather > high motivation to go ahead with at least the first patch. I totally agree with the benefits, I just want everyone to be honest and clear about the performance hit with native AGP Radeons, which already have very weak performance by today's standards even with AGP. -- Earthling Michel Dänzer | https://redhat.com Libre software enthusiast | Mesa and X developer _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
