drm/i915: Avoid context dereference inside execlists_submission_tasklet

A lesson that has to be relearnt over and over again is that the request
does not keep a reference to the context and so we cannot freely
dereference the context from inside the execlists_submission_tasklet. In
particular, we try to do so in the new GEM_TRACE() so convert those over
to the port->context_id we keep for GEM debugging. This means the
tracing now depends on DRM_I915_GEM_DEBUG.

Fixes: bccd3b831185 ("drm/i915: Use trace_printk to provide a death rattle for GEM")
References: https://bugs.freedesktop.org/show_bug.cgi?id=104066
References: https://bugs.freedesktop.org/show_bug.cgi?id=104162
References: https://bugs.freedesktop.org/show_bug.cgi?id=104242
References: https://bugs.freedesktop.org/show_bug.cgi?id=104310
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171219220916.30882-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/Kconfig.debug

@@ -29,7 +29,6 @@ config DRM_I915_DEBUG
 	select SW_SYNC # signaling validation framework (igt/syncobj*)
 	select DRM_I915_SW_FENCE_DEBUG_OBJECTS
 	select DRM_I915_SELFTEST
-	select DRM_I915_TRACE_GEM
         default n
         help
           Choose this option to turn on extra driver debugging that may affect
@@ -53,6 +52,7 @@ config DRM_I915_DEBUG_GEM
 
 config DRM_I915_TRACE_GEM
 	bool "Insert extra ftrace output from the GEM internals"
+	depends on DRM_I915_DEBUG_GEM
 	select TRACING
 	default n
 	help

drivers/gpu/drm/i915/intel_lrc.c

@@ -449,7 +449,7 @@ static void execlists_submit_ports(struct intel_engine_cs *engine)
 
 			GEM_TRACE("%s in[%d]:  ctx=%d.%d, seqno=%x\n",
 				  engine->name, n,
-				  rq->ctx->hw_id, count,
+				  port[n].context_id, count,
 				  rq->global_seqno);
 		} else {
 			GEM_BUG_ON(!n);
@@ -861,7 +861,7 @@ static void execlists_submission_tasklet(unsigned long data)
 			 */
 
 			status = READ_ONCE(buf[2 * head]); /* maybe mmio! */
-			GEM_TRACE("%s csb[%dd]: status=0x%08x:0x%08x\n",
+			GEM_TRACE("%s csb[%d]: status=0x%08x:0x%08x\n",
 				  engine->name, head,
 				  status, buf[2*head + 1]);
 
@@ -905,7 +905,7 @@ static void execlists_submission_tasklet(unsigned long data)
 			rq = port_unpack(port, &count);
 			GEM_TRACE("%s out[0]: ctx=%d.%d, seqno=%x\n",
 				  engine->name,
-				  rq->ctx->hw_id, count,
+				  port->context_id, count,
 				  rq->global_seqno);
 			GEM_BUG_ON(count == 0);
 			if (--count == 0) {