[Intel-gfx] [PATCH] drm/i915: use a separate context for gpu relocs

[Daniele Ceraolo Spurio]

The CS pre-parser can pre-fetch commands across memory sync points and
starting from gen12 it is able to pre-fetch across BB_START and BB_END
boundaries as well, so when we emit gpu relocs the pre-parser might
fetch the target location of the reloc before the memory write lands.

The parser can't pre-fetch across the ctx switch, so we use a separate
context to guarantee that the memory is syncronized before the parser
can get to it.

Suggested-by: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
---
 .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 27 ++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index b5f6937369ea..d27201c654e9 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -252,6 +252,7 @@ struct i915_execbuffer {
 		bool has_fence : 1;
 		bool needs_unfenced : 1;
 
+		struct intel_context *ce;
 		struct i915_request *rq;
 		u32 *rq_cmd;
 		unsigned int rq_size;
@@ -903,6 +904,7 @@ static void reloc_cache_init(struct reloc_cache *cache,
 	cache->has_fence = cache->gen < 4;
 	cache->needs_unfenced = INTEL_INFO(i915)->unfenced_needs_alignment;
 	cache->node.allocated = false;
+	cache->ce = NULL;
 	cache->rq = NULL;
 	cache->rq_size = 0;
 }
@@ -943,6 +945,7 @@ static void reloc_gpu_flush(struct reloc_cache *cache)
 static void reloc_cache_reset(struct reloc_cache *cache)
 {
 	void *vaddr;
+	struct intel_context *ce;
 
 	if (cache->rq)
 		reloc_gpu_flush(cache);
@@ -973,6 +976,10 @@ static void reloc_cache_reset(struct reloc_cache *cache)
 		}
 	}
 
+	ce = fetch_and_zero(&cache->ce);
+	if (ce)
+		intel_context_put(ce);
+
 	cache->vaddr = 0;
 	cache->page = -1;
 }
@@ -1168,7 +1175,7 @@ static int __reloc_gpu_alloc(struct i915_execbuffer *eb,
 	if (err)
 		goto err_unmap;
 
-	rq = i915_request_create(eb->context);
+	rq = intel_context_create_request(cache->ce);
 	if (IS_ERR(rq)) {
 		err = PTR_ERR(rq);
 		goto err_unpin;
@@ -1239,6 +1246,24 @@ static u32 *reloc_gpu(struct i915_execbuffer *eb,
 		if (!intel_engine_can_store_dword(eb->engine))
 			return ERR_PTR(-ENODEV);
 
+		/*
+		 * The CS pre-parser can pre-fetch commands across memory sync
+		 * points and starting gen12 it is able to pre-fetch across
+		 * BB_START and BB_END boundaries (within the same context). We
+		 * use a separate context to guarantee that the reloc writes
+		 * land before the parser gets to the target memory location.
+		 */
+		if (!cache->ce) {
+			struct intel_context *ce;
+
+			ce = intel_context_create(eb->context->gem_context,
+						  eb->engine);
+			if (IS_ERR(ce))
+				return ERR_CAST(ce);
+
+			cache->ce = ce; /* released in reloc_cache_reset */
+		}
+
 		err = __reloc_gpu_alloc(eb, vma, len);
 		if (unlikely(err))
 			return ERR_PTR(err);
-- 
2.23.0