[Nouveau] [PATCH/TESTING(all hw)/DISCUSSION] FIFO (minor) create and (major) destroy instabilities on nv50+

[Maarten Maathuis]

Many people using nv50+ hardware are aware of gpu lockups when a fifo
closes under certain conditions. Based on a mmio-trace and some trail
and error testing i've come up with a patch that improves the
situation on my NV96.

This patch needs testing on NV50+ hardware and regression testing on
older hardware, since i did change some of the common codepaths. This
is very much a work in progress, and if you have anything to
add/correct, please share it.

I've also attached a 2 test apps, once is bitscan-fail from mwk, use
it like ./bitscan-fail 0x200 to trigger PGRAPH errors. A modified
version only emits NOPs (method 0x100) and represents the no error
situation.

For me, i can run the NOP program in loops of 10000 iterations with no
problems (i've done so several times), the bitscan-fail survives 10000
iterations sometimes, but can also fail after a few thousand. In
comparison, a single run of bitscan-fail could cause a gpu lockup for
me in the past.

Please try the gallium driver, the test apps, suspend to ram. Suspend
to ram isn't 100% reliable yet for me (this was always the case after
strange experiments/hammering/etc), but should not regress. This goes
for older hw as well, whatever worked should still work, but i
wouldn't expect serious improvements there.

As always, feedback is appreciated, especially since this is a touchy subject.

Maarten.
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