graphics on G4

[Justin Hibbits]

On Sat, Mar 07, 2009 at 03:23:54PM -0600, Nathan Whitehorn wrote:
> Justin Hibbits wrote:
> > On Fri, Mar 06, 2009 at 07:06:26AM -0500, Justin Hibbits wrote:
> >>>>> 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.
> >>> My card, at least, has a framebuffer BAR that is 128 MB long, but only 
> >>> actually has 32 MB of graphics RAM. Writing anything to that 32 MB does 
> >>> not cause problems, but of course writing beyond that kills the machine, 
> >>> since that memory region does not actually exist. Is this true for yours 
> >>>   as well?
> >>> -Nathan
> >> My card has 256MB of graphics RAM, and according to dmesg allocates that size
> >> block, plus 64k in the 256MB region before it.  I haven't yet tested what
> >> address byte actually causes the crash, but that can be determined relatively
> >> easily.  What I'm guessing, though, is writing to anything past (end - 32k) will
> >> cause the crash.  I'll see if I can test this weekend.
> >>
> >> So, short answer to your question: it's not quite true for me, because it
> >> happens when I write to what should be inside graphics RAM.
> >>
> >> - Justin
> > 
> > Nathan-
> > 
> > Attached is a simple program I wrote that just writes a straight line down the
> > first column of the framebuffer.  It reliably kills my system while running a
> > 'make buildworld' within a few minutes of running this program.
> > 
> > (Possibly) Relevant Details:
> > 
> > ATI Radeon 9600PRO, 256MB edition.
> > 1280x1024x8 framebuffer
> > 1.25GiB RAM
> > 
> > - Justin
> > 
> 
> Thanks for the test. It looks like you've exposed a bug in how we mmap() 
> physical memory. It seems that if you try to write to the last page of 
> any mmap()ed physmem region, you get a panic in UMA. Using write(), on 
> the other hand, works just fine.
> -Nathan

Thanks for finding the actual problem.  I'll be a welcome guinea pig for testing
any patch for this.

- Justin