On Tue, May 12, 2020 at 12:38 PM Daniel Vetter <daniel@xxxxxxxx> wrote: > > On Tue, May 12, 2020 at 3:22 PM Alex Deucher <alexdeucher@xxxxxxxxx> wrote: > > > > On Tue, May 12, 2020 at 5:40 AM Karoly Balogh (Charlie/SGR) > > <charlie@xxxxxxxxxxxxxxxx> wrote: > > > > > > Hi, > > > > > > On Tue, 12 May 2020, Rui Salvaterra wrote: > > > > > > > > FWIW, on my last-generation PowerBook with RV350 (IIRC), there was a > > > > > big performance difference between AGP and PCI GART. The latter was > > > > > sort of usable for normal desktop operation, but not so much for > > > > > OpenGL apps (which were usable with AGP). > > > > > > > > I never really understood what were the issues with AGP on PowerPC > > > > (well, Apple, the only ones I've tested) machines. I mean, did OS X also > > > > disable AGP entirely, or did it have workarounds somewhere else on the > > > > stack nobody was able to figure out? > > > > > > I don't know about OS X, but I doubt there is a major/blocker hardware > > > issue, at least not one which affects every AGP machine. > > > > > > MorphOS' own Radeon driver uses the AGP facilities to some degree on all > > > AGP PowerPC Macs supported by that OS, which is from PMac AGP Graphics > > > (3,1) all the way up to the AGP G5 (7,3), including the various portables > > > and the Mac mini G4. For example it can utilize it to stream video data > > > directly from mainboard RAM, so you don't have to copy it with the CPU, > > > allowing reasonably good 720p h264 video playback on most systems above > > > the 1Ghz mark with the native MPlayer port. I'm sure the 3D part of the > > > driver also use it to some degree, given the performance improvement we > > > experienced when the AGP support was enabled (initially the system was > > > running without it), but to which extent I can't say. > > > > The problem is AGP doesn't support CPU cache snooping. Technically > > PCI must support coherent device access to system memory. Unsnooped > > access is an optional feature and some platforms may not support it at > > all. Unfortunately, AGP required unsnooped access. x8t generally > > provides a way to do this, but other platforms, not so much. I don't > > recall to what extent PowerPC supported this. The Linux DMA API > > doesn't really have a way to get uncached memory for DMA so there is > > that too. Windows and Mac may provide a way to do this depending on > > the platforms. What probably should have been done on AGP boards was > > to use both the AGP GART and the device GART. The former for uncached > > memory (if the platform supported it) and the latter for cached > > memory. That never happened. > > Slight correction on the dma-api side of things: The dma-api very much > can give you uncached memory, but only on some platforms, and the > dma-api is very opinionated about which those are. And it refuses to > tell you whether your memory ends up being uncached or cached. That's > all done in the name of platform portability, which is good for most > drivers, but just too much pain for gpu drivers. Out of curiosity how do you do that without manually messing around with PAT or MTRRs? Alex > > Otherwise all agree, agp is a mighty mess and essentially just > crapshot outside of x86. It kinda worked for the much more static > allocations for dri1, but with in-kernel memory managers all the cache > flushing issues showed up big time and it all fell to pieces. Plus a > lot of these host chipset back then where designed for the rather > static windows gpu managers, so even on x86 the coherency issues for > agp mode when used together with ttm or something else really dynamic > is pretty bad because the hw just doesn't really cope and has all > kinds of flushing troubles and races. I think the later agp chipsets > were better. > -Daniel > -- > Daniel Vetter > Software Engineer, Intel Corporation > +41 (0) 79 365 57 48 - http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
