drm/i915/execlists: Explicitly reset both reg and context runtime

Upon a GPU reset, we copy the default context image over top of the
guilty image. This will rollback the CTX_TIMESTAMP register to before
our value of ce->runtime.last. Reset both back to 0 so that we do not
encounter an underflow on the next schedule out after resume.

This should not be a huge issue in practice, as hangs should be rare in
correct code.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200330125827.5804-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -238,6 +238,17 @@ __execlists_update_reg_state(const struct intel_context *ce,
 			     const struct intel_engine_cs *engine,
 			     u32 head);
 
+static u32 intel_context_get_runtime(const struct intel_context *ce)
+{
+	/*
+	 * We can use either ppHWSP[16] which is recorded before the context
+	 * switch (and so excludes the cost of context switches) or use the
+	 * value from the context image itself, which is saved/restored earlier
+	 * and so includes the cost of the save.
+	 */
+	return READ_ONCE(ce->lrc_reg_state[CTX_TIMESTAMP]);
+}
+
 static void mark_eio(struct i915_request *rq)
 {
 	if (i915_request_completed(rq))
@@ -1154,6 +1165,7 @@ static void restore_default_state(struct intel_context *ce,
 		       engine->context_size - PAGE_SIZE);
 
 	execlists_init_reg_state(regs, ce, engine, ce->ring, false);
+	ce->runtime.last = intel_context_get_runtime(ce);
 }
 
 static void reset_active(struct i915_request *rq,
@@ -1195,17 +1207,6 @@ static void reset_active(struct i915_request *rq,
 	ce->lrc_desc |= CTX_DESC_FORCE_RESTORE;
 }
 
-static u32 intel_context_get_runtime(const struct intel_context *ce)
-{
-	/*
-	 * We can use either ppHWSP[16] which is recorded before the context
-	 * switch (and so excludes the cost of context switches) or use the
-	 * value from the context image itself, which is saved/restored earlier
-	 * and so includes the cost of the save.
-	 */
-	return READ_ONCE(ce->lrc_reg_state[CTX_TIMESTAMP]);
-}
-
 static void st_update_runtime_underflow(struct intel_context *ce, s32 dt)
 {
 #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
@@ -4614,6 +4615,7 @@ static void init_common_reg_state(u32 * const regs,
 	regs[CTX_CONTEXT_CONTROL] = ctl;
 
 	regs[CTX_RING_CTL] = RING_CTL_SIZE(ring->size) | RING_VALID;
+	regs[CTX_TIMESTAMP] = 0;
 }
 
 static void init_wa_bb_reg_state(u32 * const regs,