Re: [PATCH igt] lib: Skip aliased bsd ABI ring if bsd2 is available

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 21/02/2018 11:21, 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.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@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..868d68f7 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) {

What is this testing for? Why not just ring & i915_EXEC_RING_MASK == 
I915_EXEC_BSD? It there are some other bits set it will behave strangely.

> -		if (ring & (3 << 13) && !gem_has_bsd2(fd))

Disallow explicit selection if no vcs2, makes sense.

> +		if (!(ring & (3 << 13)) ^ gem_has_bsd2(fd))

Ugh..

If default BSD (1)
	and no BSD2 -> 1 ^ 0 = 1 OK
	and BSD2 -> 1 ^ 1 = 1 BAD

If explicit BSD (0)
	and no BSD2 -> 0 ^ 0 = 0 BAD
	has BSD2 -> 0 ^ 1 = 1 = OK

Or I made a mistake, its possible..

Regards,

Tvrtko

>   			return false;
>   	}
>   
> 

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