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

[Antonio Argenziano <antonio.argenziano@intel.com>]

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@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@intel.com>

> -Chris
> 
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