[igt-dev] [PATCH i-g-t] i915/gem_ctx_isolation: Ignore the low bits of BB_OFFSET

[Chris Wilson]

Quoting Chris Wilson (2019-01-10 21:27:54)
> Quoting Antonio Argenziano (2019-01-10 21:24:56)
> > 
> > 
> > On 07/01/19 04:41, Chris Wilson wrote:
> > > On Skylake, BB_OFFSET seems to be unstable. Since this is an
> > > offset into the batch at the time of CS execution, it should be actively
> > > written to as we read from the register so allow it a qword of
> > > discrepancy (since the CS should be reading in qwords). This still
> > > allows us to detect dirt across the rest of the register field, should
> > > that be required.
> > > 
> > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > ---
> > >   tests/i915/gem_ctx_isolation.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/tests/i915/gem_ctx_isolation.c b/tests/i915/gem_ctx_isolation.c
> > > index 058cf3ec1..78a244382 100644
> > > --- a/tests/i915/gem_ctx_isolation.c
> > > +++ b/tests/i915/gem_ctx_isolation.c
> > > @@ -96,7 +96,7 @@ static const struct named_register {
> > >       { "GPGPU_THREADS_DISPATCHED", GEN8, RCS0, 0x2290, 2 },
> > >       { "PS_INVOCATION_COUNT_1", GEN8, RCS0, 0x22f0, 2 },
> > >       { "PS_DEPTH_COUNT_1", GEN8, RCS0, 0x22f8, 2 },
> > > -     { "BB_OFFSET", GEN8, RCS0, 0x2158 },
> > > +     { "BB_OFFSET", GEN8, RCS0, 0x2158, .ignore_bits = 0x7 },
> > 
> > The batch offset starts at bit 2. Do we observe changes in bit 0-1 as well?
> 
> Not, it is just off by bit 2 (0x4). Bit 0 is also set when I don't
> really expect it to be, I guess I really should just read what the
> register is meant to be rather than guessing solely on the basis of its
> name.

Bit 2 flip flops between reference value and observed (test failure).

Bit 0 simply differs from my own expectations.
-Chris