drm/i915: Filter out spurious execlists context-switch interrupts

Back in commit a4b2b01523a8 ("drm/i915: Don't mark an execlists
context-switch when idle") we noticed the presence of late
context-switch interrupts. We were able to filter those out by looking
at whether the ELSP remained active, but in commit beecec901790
("drm/i915/execlists: Preemption!") that became problematic as we now
anticipate receiving a context-switch event for preemption while ELSP
may be empty. To restore the spurious interrupt suppression, add a
counter for the expected number of pending context-switches and skip if
we do not need to handle this interrupt to make forward progress.

v2: Don't forget to switch on for preempt.
v3: Reduce the counter to a on/off boolean tracker. Declare the HW as
active when we first submit, and idle after the final completion event
(with which we confirm the HW says it is idle), and track each source
of activity separately. With a finite number of sources, it should aide
us in debugging which gets stuck.

Fixes: beecec901790 ("drm/i915/execlists: Preemption!")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Michal Winiarski <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Arkadiusz Hiler <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Mika Kuoppala <[email protected]>

1 files changed, 32 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 17186f067408..6a42ed618a28 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -241,9 +241,17 @@ struct intel_engine_execlists {
 	} port[EXECLIST_MAX_PORTS];
 
 	/**
-	 * @preempt: are we currently handling a preempting context switch?
+	 * @active: is the HW active? We consider the HW as active after
+	 * submitting any context for execution and until we have seen the
+	 * last context completion event. After that, we do not expect any
+	 * more events until we submit, and so can park the HW.
+	 *
+	 * As we have a small number of different sources from which we feed
+	 * the HW, we track the state of each inside a single bitfield.
 	 */
-	bool preempt;
+	unsigned int active;
+#define EXECLISTS_ACTIVE_USER 0
+#define EXECLISTS_ACTIVE_PREEMPT 1
 
 	/**
 	 * @port_mask: number of execlist ports - 1
@@ -525,6 +533,27 @@ struct intel_engine_cs {
 	u32 (*get_cmd_length_mask)(u32 cmd_header);
 };
 
+static inline void
+execlists_set_active(struct intel_engine_execlists *execlists,
+		     unsigned int bit)
+{
+	__set_bit(bit, (unsigned long *)&execlists->active);
+}
+
+static inline void
+execlists_clear_active(struct intel_engine_execlists *execlists,
+		       unsigned int bit)
+{
+	__clear_bit(bit, (unsigned long *)&execlists->active);
+}
+
+static inline bool
+execlists_is_active(const struct intel_engine_execlists *execlists,
+		    unsigned int bit)
+{
+	return test_bit(bit, (unsigned long *)&execlists->active);
+}
+
 static inline unsigned int
 execlists_num_ports(const struct intel_engine_execlists * const execlists)
 {
@@ -538,6 +567,7 @@ execlists_port_complete(struct intel_engine_execlists * const execlists,
 	const unsigned int m = execlists->port_mask;
 
 	GEM_BUG_ON(port_index(port, execlists) != 0);
+	GEM_BUG_ON(!execlists_is_active(execlists, EXECLISTS_ACTIVE_USER));
 
 	memmove(port, port + 1, m * sizeof(struct execlist_port));
 	memset(port + m, 0, sizeof(struct execlist_port));