drm/i915: Disable use of hwsp_cacheline for kernel_context

Currently on execlists, we use a local hwsp for the kernel_context,
rather than the engine's HWSP, as this is the default for execlists.
However, seqno wrap requires allocating a new HWSP cacheline, and may
require pinning a new HWSP page in the GGTT. This operation requiring
pinning in the GGTT is not allowed within the kernel_context timeline,
as doing so may require re-entering the kernel_context in order to evict
from the GGTT. As we want to avoid requiring a new HWSP for the
kernel_context, we can use the permanently pinned engine's HWSP instead.
However to do so we must prevent the use of semaphores reading the
kernel_context's HWSP, as the use of semaphores do not support rollover
onto the same cacheline. Fortunately, the kernel_context is mostly
isolated, so unlikely to give benefit to semaphores.

Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200210205722.794180-5-chris@chris-wilson.co.uk

1 files changed, 12 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 70d91ad923ef..902d440ef07d 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -2964,7 +2964,8 @@ static int gen8_emit_init_breadcrumb(struct i915_request *rq)
 {
 	u32 *cs;
 
-	GEM_BUG_ON(!i915_request_timeline(rq)->has_initial_breadcrumb);
+	if (!i915_request_timeline(rq)->has_initial_breadcrumb)
+		return 0;
 
 	cs = intel_ring_begin(rq, 6);
 	if (IS_ERR(cs))
@@ -4616,8 +4617,17 @@ static int __execlists_context_alloc(struct intel_context *ce,
 
 	if (!ce->timeline) {
 		struct intel_timeline *tl;
+		struct i915_vma *hwsp;
+
+		/*
+		 * Use the static global HWSP for the kernel context, and
+		 * a dynamically allocated cacheline for everyone else.
+		 */
+		hwsp = NULL;
+		if (unlikely(intel_context_is_barrier(ce)))
+			hwsp = engine->status_page.vma;
 
-		tl = intel_timeline_create(engine->gt, NULL);
+		tl = intel_timeline_create(engine->gt, hwsp);
 		if (IS_ERR(tl)) {
 			ret = PTR_ERR(tl);
 			goto error_deref_obj;