[Intel-gfx] [PATCH] drm/i915: Do not add an interrupt for a context switch

[Chris Wilson]

We use the request to ensure we hold a reference to the context for the
duration that it remains in use by the ring. Each request only holds a
reference to the current context, hence we emit a request after
switching contexts with the final reference to the old context. However,
the extra interrupt caused by that request is not useful (no timing
critical function will wait for the context object), instead the overhead
of servicing the IRQ shows up in some (lightweight) benchmarks. In order
to keep the useful property of using the request to manage the context
lifetime, we want to add a dummy request that is associated with the
interrupt from the subsequent real request following the batch.

The extra interrupt was added as a side-effect of using
i915_add_request() in

commit 112522f6789581824903f6f72082b5b841a7f0f9
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Thu May 2 16:48:07 2013 +0300

    drm/i915: put context upon switching

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ben Widawsky <ben at bwidawsk.net>
Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
---
 drivers/gpu/drm/i915/i915_drv.h         |  1 +
 drivers/gpu/drm/i915/i915_gem.c         | 32 +++++++++++++++++++++++++++++++-
 drivers/gpu/drm/i915/i915_gem_context.c |  2 +-
 3 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 9a8fac3..e822390 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1873,6 +1873,7 @@ int __i915_add_request(struct intel_ring_buffer *ring,
 		       struct drm_file *file,
 		       struct drm_i915_gem_object *batch_obj,
 		       u32 *seqno);
+int __i915_add_request_marker(struct intel_ring_buffer *ring);
 #define i915_add_request(ring, seqno) \
 	__i915_add_request(ring, NULL, NULL, seqno)
 int __must_check i915_wait_seqno(struct intel_ring_buffer *ring,
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 0fa45e0..7568bf0 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2110,6 +2110,37 @@ i915_gem_get_seqno(struct drm_device *dev, u32 *seqno)
 	return 0;
 }
 
+int
+__i915_add_request_marker(struct intel_ring_buffer *ring)
+{
+	struct drm_i915_gem_request *request;
+
+	request = kmalloc(sizeof(*request), GFP_KERNEL);
+	if (request == NULL)
+		return -ENOMEM;
+
+	request->ring = ring;
+	request->seqno = intel_ring_get_seqno(ring);
+	request->tail = request->head = intel_ring_get_tail(ring);
+	request->batch_obj = NULL;
+
+	/* Whilst this request exists, batch_obj will be on the
+	 * active_list, and so will hold the active reference. Only when this
+	 * request is retired will the the batch_obj be moved onto the
+	 * inactive_list and lose its active reference. Hence we do not need
+	 * to explicitly hold another reference here.
+	 */
+	request->ctx = ring->last_context;
+	if (request->ctx)
+		i915_gem_context_reference(request->ctx);
+
+	request->emitted_jiffies = jiffies;
+	list_add_tail(&request->list, &ring->request_list);
+	request->file_priv = NULL;
+
+	return 0;
+}
+
 int __i915_add_request(struct intel_ring_buffer *ring,
 		       struct drm_file *file,
 		       struct drm_i915_gem_object *obj,
@@ -2137,7 +2168,6 @@ int __i915_add_request(struct intel_ring_buffer *ring,
 	if (request == NULL)
 		return -ENOMEM;
 
-
 	/* Record the position of the start of the request so that
 	 * should we detect the updated seqno part-way through the
 	 * GPU processing the request, we never over-estimate the
diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index db94aca..88a9425 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -466,7 +466,7 @@ static int do_switch(struct i915_hw_context *to)
 		from->obj->dirty = 1;
 		BUG_ON(from->obj->ring != ring);
 
-		ret = i915_add_request(ring, NULL);
+		ret = __i915_add_request_marker(ring);
 		if (ret) {
 			/* Too late, we've already scheduled a context switch.
 			 * Try to undo the change so that the hw state is
-- 
1.8.4.rc1