[Intel-gfx] [PATCH igt v2] lib: Skip aliased bsd ABI ring if bsd2 is available

[Tvrtko Ursulin]

On 21/02/2018 13:07, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-02-21 12:55:17)
>>
>> On 21/02/2018 12:53, Tvrtko Ursulin wrote:
>>>
>>> On 21/02/2018 12:17, Chris Wilson wrote:
>>>> How much do I want this uABI to rot away? Say "Never again!" to implicit
>>>> aliasing.
>>>>
>>>> In the meantime, we do not need to perform duplicate work on bsd2
>>>> machines, as especially we do not know which engine bsd relates to.
>>>>
>>>> v2: When in doubt, shout!
>>>>
>>>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>>> ---
>>>>    lib/ioctl_wrappers.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/lib/ioctl_wrappers.c b/lib/ioctl_wrappers.c
>>>> index 8748cfcf..b9b86079 100644
>>>> --- a/lib/ioctl_wrappers.c
>>>> +++ b/lib/ioctl_wrappers.c
>>>> @@ -1460,7 +1460,7 @@ bool gem_has_ring(int fd, unsigned ring)
>>>>        /* silly ABI, the kernel thinks everyone who has BSD also has
>>>> BSD2 */
>>>>        if ((ring & ~(3<<13)) == I915_EXEC_BSD) {
>>>> -        if (ring & (3 << 13) && !gem_has_bsd2(fd))
>>>> +        if (!(ring & (3 << 13)) ^ !gem_has_bsd2(fd))
>>>>                return false;
>>>>        }
>>>
>>> If default BSD (1)
>>>       and no BSD2 -> 1 ^ 1 = 1 OK
>>
>> Oops
>> 1 ^ 1 = 0 so also bad
>>
>>>       and BSD2 -> 1 ^ 0 = 1 BAD
>>>
>>> If explicit BSD (0)
>>>       and no BSD2 -> 0 ^ 1 = 1 BAD
>>>       has BSD2 -> 0 ^ 0 = 0 = BAD
>>>
>>> Maybe I'm confused.. please simplify the statement? :)
> 
> return false?
> 
> default BSD ^ has-bsd2
> 0		0	-> 0 OK
> 0		1	-> 1 SKIP
> 1		0	-> 1 SKIP
> 1		1	-> 0 OK
> 
> So we only use bsd (implicit) on single BSD engine machines, and only
> use bsd0,bsd1 (explicit) on dual engine machines.

Don't get in your notation why is 0 OK and 1 SKIP. 0 is false = no ring 
= skip.

default BSD ^ 	has-bsd2
0		0		0 ^ !0 = 1 has engine - WRONG
0		1		0 ^ !1 = 0 no engine - WRONG
1		0		1 ^ !0 = 0 no engine - WRONG
1		1		1 ^ !1 = 1 has engine - WRONG


default BSD ^ 	has-bsd2
0		0		0 ^ 0 = 0 no engine - OK
0		1		0 ^ 1 = 1 has engine - OK
1		0		1 ^ 0 = 1 has engine - OK
1		1		1 ^ 1 = 0 no engine - OK

So previous version was OK!?

Regards,

Tvrtko