drm/i915: Add comment how we treat hung contexts

	/* Read once and return the resolution */

	const bool guilty = engine_stalled(request->engine);

	/* The guilty request will get skipped on a hung engine.

	 *

	 * Users of client default contexts do not rely on logical

	 * state preserved between batches so it is safe to execute

	 * queued requests following the hang. Non default contexts

	 * rely on preserved state, so skipping a batch loses the

	 * evolution of the state and it needs to be considered corrupted.

	 * Executing more queued batches on top of corrupted state is

	 * risky. But we take the risk by trying to advance through

	 * the queued requests in order to make the client behaviour

	 * more predictable around resets, by not throwing away random

	 * amount of batches it has prepared for execution. Sophisticated

	 * clients can use gem_reset_stats_ioctl and dma fence status

	 * (exported via sync_file info ioctl on explicit fences) to observe

	 * when it loses the context state and should rebuild accordingly.

	 *

	 * The context ban, and ultimately the client ban, mechanism are safety

	 * valves if client submission ends up resulting in nothing more than

	 * subsequent hangs.

	 */

	if (guilty) {

		i915_gem_context_mark_guilty(request->ctx);

		skip_request(request);