[Intel-gfx] I've got the RC6 bug

[Daniel Vetter]

On Wed, Jan 18, 2012 at 09:51:30AM -0800, Eric Anholt wrote:
> On Wed, 18 Jan 2012 11:17:52 +0000, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > On Wed, 18 Jan 2012 01:24:26 +0100, Daniel Vetter <daniel at ffwll.ch> wrote:
> > > On Wed, Jan 18, 2012 at 01:16:02AM +0100, CC wrote:
> > > > I attached the error state.
> > > 
> > > Nice one, your gpu seems to have simply disappeared. And the ringbuffer
> > > contains a rather peculiar cmd sequence. Putting Chris (maybe he
> > > recognizes the pattern) and Ben (he's got a patch in the works to dump a
> > > debug register that might be interesting here) on cc. It's too late atm
> > > for me to think about this some more.
> > 
> > Not simply disappeared, someone clobbered it with an extremely large
> > hammer. The GPU was killed by a stray write to address 0 which took out
> > the render ring buffer and its hws page. So my first thought is a
> > missing relocation, and i965g springs to mind.
> > -Chris
> 
> At one point there was a bug in Mesa that wrote to 0:
> 
> commit dfada714f8db3deea2fea3583c3c166a78db1117
> Author: Eric Anholt <eric at anholt.net>
> Date:   Fri Jun 17 18:20:36 2011 -0700
> 
>     i965/gen6: Use an BO instead of writing to address 0 for PIPE_CONTROL W/A.
>     
>     This was spectacularly unsafe.  On my system, address 0 happens to be
>     the hardware status page for the render ring, and the first quadword
>     of that happens to contain nothing we ever look at, but I sure didn't
>     look forward to having to debug some day when, for example, the kernel
>     happened to bind the ringbuffer before binding the hwsp.

Unfortunately the error_state contains more garbage than just one stray 0
write. So yeah, if this is due to the i965g gallium driver, that would
explain things - otherwise I'm hoping for Ben's reworked gt fifo patch.
The CS regs are all 0, indicating that the gpu isn't getting out of deep
sleep anymore.
-Daniel
-- 
Daniel Vetter
Mail: daniel at ffwll.ch
Mobile: +41 (0)79 365 57 48