[Intel-gfx] [PATCH v3 13/21] drm/i915: Replace the pinned context address with its unique ID

[Dave Gordon]

On 25/04/16 16:48, Chris Wilson wrote:
> On Mon, Apr 25, 2016 at 04:21:58PM +0100, Dave Gordon wrote:
>> On 24/04/16 19:10, Chris Wilson wrote:
>>> Rather than reuse the current location of the context in the global GTT
>>> for its hardware identifier, use the context's unique ID assigned to it
>>> for its whole lifetime.
>>>
>>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>>> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> Hmmm I wonder whether this will work with the GuC?
>> IIRC the GuC firmware requires the LRCA to be used as the CTX ID :(
>> Or at least it used to, maybe that has changed now.
>
> I know you raised it last time, as far as I can tell the two are
> distinct in i915_guc_submission and you never pass the high 32 bits of
> the LRC context descriptor (the context id) to the GuC. i.e.
>
> guc_init_ctx_desc:
> 	ctx_desc = intel_lr_context_descriptor(ctx, engine);
> 	lrc->context_desc = (u32)ctx_desc;
>
> 	lrc->ring_lrca = i915_gem_object_gtt_offset(ctx_obj) + PAGE;
> 	lrc->context_id = client->ctx_index | engine->guc_id;
>
> The guc hasn't fared any worse then it did before applying the change...
> -Chris

I found a comment in the GuC firmware,

// SubmissionByProxy: if KMD or other context submitted this
//		context. This means, ContextID is LRCA[31:20]

But it turns out that's referring to the field that we always set to the 
LRCA anyway (since we DO use submission by proxy), but which would be 
used for other purposes in some other mode. So all OK :)

Also, I've tested this patchset on SKL and we're not getting any 
complaints from the GuC :)

One other thing I've noticed in the BSpec: it says the "s/w context id" 
is 21 bits rather than 20 (bits 52-32 inclusive). Then bits 53-54 are 
MBZ, and then the remaining 9 bits (55-63) are "GroupID" which seems to 
be a per-process identifier which is *also* required to be unique among 
active contexts. But maybe that's only when taken in combination with 
the LRCA, which is automatically unique among active contexts anyway?

.Dave.