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

[Chris Wilson]

Quoting Antonio Argenziano (2019-01-10 21:58:52)
> 
> 
> On 10/01/19 13:29, Chris Wilson wrote:
> > 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.
> 
> I guess if it gets overwritten we catch it even if we ignore the lowest 
> 3 bits but something weird would have happened if 0-1 change.
> 
> With or without modifying the mask,
> Reviewed-by: Antonio Argenziano <antonio.argenziano at intel.com>

Restricted the mask to .ignore_bits=0x4 so that we only ignore the bit
that is fluctuating in testing.

Thanks,
-Chris