[igt-dev] [PATCH i-g-t] igt/gem_exec_await: Tag the final batch in the GTT

[Chris Wilson]

Quoting Katarzyna Dec (2018-06-08 08:36:35)
> On Thu, Jun 07, 2018 at 04:27:22PM +0100, Chris Wilson wrote:
> > Batches are contained in their position within the GTT by the kernel,
> > and if they are in an invalid poistion will be unbound and rebound
> > before execution. In our test setup, we therefore need to place the
> > batch into a valid poistion within the GTT before we fill the ring with
> > busyspinners.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Reviewed-by: Katarzyna Dec <katarzyna.dec at intel.com>

Sorry for being curt on irc, not enough coffee.

The problem entirely lies in how we are constructing our set of busy
spinning batches. We try to fill the ring with a chain of batches that
are all linked to one buffer, and then try to execute that buffer. This
gives us the most implicit fences on that one buffer we can trivially
construct; with the goal being that the kernel handles them all with
aplomb. However, what we failed to take into account was that we might
end up with that final buffer being at address 0, which is in an invalid
location to execute from (because reasons) and the kernel would be
forced to move it. However, since we have a ring full of busy spinners
all using that buffer, we can not move that buffer until we wait for the
queue to complete -- which it never will and so we declare a GPU hang,
failing the test.
-Chris