drm/i915: Measure the required reserved size for request emission

Instead of tediously and fragilely counting up the number of dwords
required to emit the breadcrumb to seal a request, fake a request and
measure it automatically once during engine setup.

The downside is that this requires a fair amount of mocking to create a
proper breadcrumb. Still, should be less error prone in future as the
breadcrumb size fluctuates!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125100520.20163-1-chris@chris-wilson.co.uk

1 files changed, 49 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
index 2f3c71f6d313..8f738a7cd117 100644
--- a/drivers/gpu/drm/i915/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/intel_engine_cs.c
@@ -604,6 +604,47 @@ static void __intel_context_unpin(struct i915_gem_context *ctx,
         intel_context_unpin(to_intel_context(ctx, engine));
 }
 
+struct measure_breadcrumb {
+        struct i915_request rq;
+        struct i915_timeline timeline;
+        struct intel_ring ring;
+        u32 cs[1024];
+};
+
+static int measure_breadcrumb_sz(struct intel_engine_cs *engine)
+{
+        struct measure_breadcrumb *frame;
+        unsigned int dw;
+
+        GEM_BUG_ON(!engine->i915->gt.scratch);
+
+        frame = kzalloc(sizeof(*frame), GFP_KERNEL);
+        if (!frame)
+                return -ENOMEM;
+
+        i915_timeline_init(engine->i915, &frame->timeline, "measure");
+
+        INIT_LIST_HEAD(&frame->ring.request_list);
+        frame->ring.timeline = &frame->timeline;
+        frame->ring.vaddr = frame->cs;
+        frame->ring.size = sizeof(frame->cs);
+        frame->ring.effective_size = frame->ring.size;
+        intel_ring_update_space(&frame->ring);
+
+        frame->rq.i915 = engine->i915;
+        frame->rq.engine = engine;
+        frame->rq.ring = &frame->ring;
+        frame->rq.timeline = &frame->timeline;
+
+        dw = engine->emit_breadcrumb(&frame->rq, frame->cs) - frame->cs;
+        GEM_BUG_ON(dw != engine->emit_breadcrumb_sz);
+
+        i915_timeline_fini(&frame->timeline);
+        kfree(frame);
+
+        return dw;
+}
+
 /**
  * intel_engines_init_common - initialize cengine state which might require hw access
  * @engine: Engine to initialize.
@@ -657,8 +698,16 @@ int intel_engine_init_common(struct intel_engine_cs *engine)
         if (ret)
                 goto err_breadcrumbs;
 
+        ret = measure_breadcrumb_sz(engine);
+        if (ret < 0)
+                goto err_status_page;
+
+        engine->emit_breadcrumb_sz = ret;
+
         return 0;
 
+err_status_page:
+        cleanup_status_page(engine);
 err_breadcrumbs:
         intel_engine_fini_breadcrumbs(engine);
 err_unpin_preempt: