[PATCH v6 22/25] drm/i915: Track the previous pinned context inside the request

Chris Wilson
2016-04-26T20:06:59Z

As the contexts are accessed by the hardware until the switch is completed
to a new context, the hardware may still be writing to the context object
after the breadcrumb is visible. We must not unpin/unbind/prune that
object whilst still active and so we keep the previous context pinned until
the following request. We can generalise the tracking we already do via
the engine->last_context and move it to the request so that it works
equally for execlists and GuC.

v2: Drop the execlists double pin as that exposes a race inside the lrc
irq handler as it tries to access the context after it may be retired.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
---
drivers/gpu/drm/i915/i915_drv.h | 11 +++++++++++
drivers/gpu/drm/i915/i915_gem.c | 8 ++++----
drivers/gpu/drm/i915/intel_lrc.c | 14 ++++++++------
3 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 0bd9d17ff974..eb4a95b853dd 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2305,6 +2305,17 @@ struct drm_i915_gem_request {
struct intel_context *ctx;
struct intel_ringbuffer *ringbuf;

+ /**
+ * Context related to the previous request.
+ * As the contexts are accessed by the hardware until the switch is
+ * completed to a new context, the hardware may still be writing
+ * to the context object after the breadcrumb is visible. We must
+ * not unpin/unbind/prune that object whilst still active and so
+ * we keep the previous context pinned until the following (this)
+ * request is retired.
+ */
+ struct intel_context *previous_context;
+
/** Batch buffer related to this request if any (used for
error state dump only) */
struct drm_i915_gem_object *batch_obj;
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index e4c8a8b998d1..fc68af50ed1b 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1413,13 +1413,13 @@ static void i915_gem_request_retire(struct drm_i915_gem_request *request)
list_del_init(&request->list);
i915_gem_request_remove_from_client(request);

- if (request->ctx) {
+ if (request->previous_context) {
if (i915.enable_execlists)
- intel_lr_context_unpin(request->ctx, request->engine);
-
- i915_gem_context_unreference(request->ctx);
+ intel_lr_context_unpin(request->previous_context,
+ request->engine);
}

+ i915_gem_context_unreference(request->ctx);
i915_gem_request_unreference(request);
}

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 4be11f8d0e04..0682a79be02c 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -771,12 +771,14 @@ intel_logical_ring_advance_and_submit(struct drm_i915_gem_request *request)
if (intel_engine_stopped(engine))
return 0;

- if (engine->last_context != request->ctx) {
- if (engine->last_context)
- intel_lr_context_unpin(engine->last_context, engine);
- intel_lr_context_pin(request->ctx, engine);
- engine->last_context = request->ctx;
- }
+ /* We keep the previous context alive until we retire the following
+ * request. This ensures that any the context object is still pinned
+ * for any residual writes the HW makes into it on the context switch
+ * into the next object following the breadcrumb. Otherwise, we may
+ * retire the context too early.
+ */
+ request->previous_context = engine->last_context;
+ engine->last_context = request->ctx;

if (dev_priv->guc.execbuf_client)
i915_guc_submit(dev_priv->guc.execbuf_client, request);