drm/i915: Add comment how we treat hung contexts

Explain in a comment how and why we treat hung context like we do.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484668747-9120-7-git-send-email-mika.kuoppala@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 3e10e810101488c1545fc8c2148a83aa07a1a4ac..d6ec63997ca3ad9bb6c38831f391b730b4be3e51 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2697,6 +2697,27 @@ static bool i915_gem_reset_request(struct drm_i915_gem_request *request)
        /* 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);