[Intel-gfx] [PATCH 1/1] drm/i915: Fix for nested_enable_signaling BUG_ON

[John Harrison]

On 11/28/2017 1:40 AM, Chris Wilson wrote:
> Quoting John.C.Harrison at Intel.com (2017-11-28 09:10:59)
>> From: John Harrison <John.C.Harrison at Intel.com>
>>
>> The call to enable signaling was occuring after the request had been
>> sent to the GuC for execution on the hardware. That means that it is
>> possible for the request to actually complete before the code to
>> enable signaling has executed.
> No, it wasn't, unless you had modified the code. Note also we switched
> to a different scheme in upstream
> -Chris

It was as of:

    commit d99c1a7aff71536e909f54709023dcc5d6b559c0
    Author: Chris Wilson <chris at chris-wilson.co.uk>
    Date:   Thu Mar 16 12:56:18 2017 +0000

    drm/i915/scheduler: emulate a scheduler for guc


The code introduced was:

    +               ...
    +               if (last && rq->ctx != last->ctx) {
    +                       ...
    *+                       nested_enable_signaling(last);**
    * +                       port++;
    +               }
    +               ...
    *+               i915_guc_submit(rq);**
    *+               last = rq;
    +               submit = true;
    +       }
    +       if (submit) {
    +               i915_gem_request_assign(&port->request, last);
    *+               nested_enable_signaling(last);**
    *+               engine->execlist_first = rb;
    +       }
    +       ...


That quite definitely calls nested_enable_signaling() after calling 
i915_guc_submit(). It is either called on the last request submitted for 
a given context or on the last request submitted overall. But in either 
case it is only called after the actual submission occurs.

Maybe that was not exactly 4.11, there might have been a few more recent 
patches pulled in to this tree. It is definitely not latest upstream 
though. And as I noted in the cover letter, moving to latest upstream is 
not an option for them at this point.

John.

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