John Richard Moser wrote: > Bernardo Innocenti wrote: > >> Tom Sylla wrote: >> >> >>> http://openbios.org/viewvc/cpu/x86/pc/olpc/lxmsrs.fth?view=markup&revision=739&root=OpenFirmware >>> has: >>> msr: 0000.1810 fdfff000.fd000111. \ Video (write through), fbsize >>> >>> which is setting the framebuffer as write-combining. (the "write >>> through" comment is incorrect) >>> >> This takes care of the physical mapping, but how would userspace >> be able to mmap the framebuffer into virtual memory without >> additional MMU programming? >> >> I was under the impression that we also need to cover the whole >> region with small 4KB MMU pages. This degrades performance >> somewhat due to TLB misses when the CPU accesses the framebuffer. >> >> > > I missed whether or not the Geode actually has 4MiB huge pages, I > thought someone said it does. This being the case, why can't you access > the 16 (or was it 24?) MiB of memory via a handful (about 1/1024th) of > huge mappings? Does x86 MMU not allow for huge MMIO? > > The Geode GX has 64 TLB entries right? I don't know how many the Geode > LX has, or if there's an L2 TLB. Obviously, though, this would be a > major performance boon, what with there being (assuming 24MiB of vram) 5 > probably often used mappings instead of 5120 in an often-used set with a > probably uneven distribution. >
The magnitude of the performance benefit is not at all obvious. The Geode's graphics accelerator uses physical addressing. > >> But I must confess I have limited understanding of the Geode >> architecture, so I may be overlooking something. >> >> > > _______________________________________________ Devel mailing list [email protected] http://lists.laptop.org/listinfo/devel
