graphics on G4

[Justin Hibbits]

On Wed, Mar 04, 2009 at 06:52:12PM -0600, Nathan Whitehorn wrote:
> Justin Hibbits wrote:
> > On Wed, Mar 04, 2009 at 09:13:10AM +1000, Peter Grehan wrote:
> >> Hi Justin,
> >>
> >>>> What happens if we have physical or device memory in the
> >>>> same range as kmem VAs?
> >>  This shouldn't happen: kmem VAs use seg regs 13 and 14 i.e.
> >> the virtually-mapped space is 0xD000.0000 -> 0xEFFF.FFFF.
> >>
> >>  Kernel physical memory is 1:1 mapped, using BAT registers, as
> >> is i/o space. Since there are only 4 BAT registers used, a DSI
> >> trap will evict a BAT and re-use it, if the faulting address
> >> falls in the battable[] array. See
> >> powerpc/aim/trap_subr.s:dsitrap().
> >>
> >>  Now, mapping the frame buffer from user-space *doesn't* use
> >> the BATs, but instead uses PTEs from user VA. Each process has
> >> unique segment register values which prevent it from
> >> corrupting memory in other address spaces (including the
> >> kernel's).
> > 
> > Seems something is overwriting kernel's memory.  Should I try simply removing
> > one of the RAM sticks and see if that fixes things?
> 
> I've just set up X on my laptop, and am seeing what I think is the same 
> problem. The system is completely stable until I start X, at which point 
> it will hang some random time later (ranging from seconds to hours), or 
> have weird panics in the UMA allocator.
> 
> This is a G4 iBook with 1.5 GB of RAM. I instrumented ofw_syscons mmap() 
> routine to check what memory X is using, and, besides the framebuffer, 
> it appears to be mapping PCI configuration space for PCI bus 1 (the one 
> that the MacIO ASIC is on, and not the one the graphics card is on). I 
> can't figure out why. There are also no memory overlaps of the 
> framebuffer -- neither with physical memory nor with KVA space.
> 
> Did you ever discover whether writing to random bits of the framebuffer 
> without ever having run X also causes this problem?
> -Nathan

Yes, I did perform that test, and it does cause the problem as well.  For me it
hangs when starting a new process some time later, so can be tested somewhat
easily by performing a buildworld after doing the graphics test.

- Justin