Re: [PATCH 24/51] drm/i915: Update deferred context creation to do explicit request management

John Harrison
2015-02-18T15:27:41Z

On 13/02/2015 12:15, Chris Wilson wrote:
> On Fri, Feb 13, 2015 at 11:48:33AM +0000, John.C.Harrison@Intel.com wrote:
>> From: John Harrison <John.C.Harrison@Intel.com>
>>
>> In execlist mode, context initialisation is deferred until first use of the
>> given context. This is because execlist mode has many more contexts than legacy
>> mode and many are never actually used.
> That's not correct. There are no more contexts in execlists than legacy.
> There are more ringbuffers, or rather the contexts have an extra state
> object associated with them.
Okay, I should have said sub-contexts. Or context state objects. Or
something.

>> Previously, the initialisation commands
>> were written to the ring and tagged with some random request structure via the
>> OLR. This seemed to be causing a null pointer deference bug under certain
>> circumstances (BZ:40112).
>>
>> This patch adds explicit request creation and submission to the deferred
>> initialisation code path. Thus removing any reliance on or randomness caused by
>> the OLR.
> This is upside down though. The request should be referencing the
> context (thus instantiating it on demand) and nothing in the context
> allocation requires the request. The initialisation here should be during
> i915_request_switch_context(), since it can be entirely shared with
> legacy.
> -Chris

The request does reference the context - the alloc_reques() function
takes a context object as a parameter. Thus it is impossible for the
request to be used/supplied/required during context creation. The issue
here is the lazy initialisation of the per ring context state which
requires sending commands to the ring on first usage of the given
context object on the given ring.

One problem is that the initialisation request and the batch buffer
request cannot be merged at the moment. They both use request->batch_obj
for tracking the command object. Thus this patch only works due to the
deferred intialisation occurring during the i915_gem_validate_context()
call very early on in execbuffer() rather than as part of the context
switch within the batch buffer execution which is much later.

I'm not sure what you mean by i915_request_switch_context(). The
existing i915_switch_context() does now take just a request structure
rather than a ring/ringbuf/context mixture. However, it is not really a
good idea to do the context switch automatically as part of creating the
request. The request creation and request execution could be quite
separated in time, especially with a scheduler.

It should be possible to move the deferred initialisation within the
context switch if the object tracking can be resolved. Thus they could
share the same request and there would not be effectively two separate
execution calls at the hardware level. Again, that's potentially work
that could be done as a follow up task of improving the context
management independent of the current task of removing the OLR.

John.