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

[Chris Wilson]

Quoting Daniele Ceraolo Spurio (2019-08-24 01:20:22)
> 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.
> +                */

I think the comment about the different semantics for execution flow
would also be valuable in emit_fini_breadcrumb -- it's the first place
we will look if we ever have any doubt about the barriers around the
breadrumbs.
-Chris