[Intel-gfx] [PATCH 06/42] drm/i915: Support asynchronous waits on struct fence from i915_gem_request

[Chris Wilson]

On Fri, Oct 07, 2016 at 04:51:14PM +0100, Tvrtko Ursulin wrote:
> 
> On 07/10/2016 10:45, Chris Wilson wrote:
> >We will need to wait on DMA completion (as signaled via struct fence)
> >before executing our i915_gem_request. Therefore we want to expose a
> >method for adding the await on the fence itself to the request.
> >
> >Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >---
> >  drivers/gpu/drm/i915/i915_gem_request.c | 40 +++++++++++++++++++++++++++++++++
> >  drivers/gpu/drm/i915/i915_gem_request.h |  2 ++
> >  2 files changed, 42 insertions(+)
> >
> >diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
> >index 8832f8ec1583..e1f7a32d4080 100644
> >--- a/drivers/gpu/drm/i915/i915_gem_request.c
> >+++ b/drivers/gpu/drm/i915/i915_gem_request.c
> >@@ -23,6 +23,7 @@
> >   */
> >  #include <linux/prefetch.h>
> >+#include <linux/fence-array.h>
> >  #include "i915_drv.h"
> >@@ -495,6 +496,45 @@ i915_gem_request_await_request(struct drm_i915_gem_request *to,
> >  	return 0;
> >  }
> >+int
> >+i915_gem_request_await_fence(struct drm_i915_gem_request *req,
> >+			     struct fence *fence)
> >+{
> >+	struct fence_array *array;
> >+	int ret;
> >+	int i;
> >+
> >+	if (test_bit(FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> >+		return 0;
> >+
> >+	if (fence_is_i915(fence))
> >+		return i915_gem_request_await_request(req, to_request(fence));
> >+
> >+	if (!fence_is_array(fence)) {
> >+		ret = i915_sw_fence_await_dma_fence(&req->submit,
> >+						    fence, 10*HZ,
> >+						    GFP_KERNEL);
> >+		return ret < 0 ? ret : 0;
> >+	}
> >+
> >+	array = to_fence_array(fence);
> >+	for (i = 0; i < array->num_fences; i++) {
> >+		struct fence *child = array->fences[i];
> 
> 1. What is the advantage in manually waiting on array elements
> rather than just the array? Is the existance of signal_on_any a
> problem?

We can wait on native fences much more efficiently. In the typical case,
we don't even need the wait at all since the ordering is imposed by the
timeline. Or we can hook into the submission fence to avoid the
interrupt. Failing that we have to respond to the interrupt. Also note
that foriegn fences handle autosignaling differently to native fences,
as we cannot rely on third parties having a working hangcheck.
 
> 2. Can child be another array?

Yes, but we don't want to recurse (or at least need to bound the
recursion).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre