[PATCH 05/16] drm/i915/ctx: Return earlier on failure

[Ben Widawsky <benjamin.widawsky@intel.com>]

As what was correctly debugged here:
commit acc240d41ea1ab9c488a79219fb313b5b46265ae
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Dec 5 15:42:34 2013 +0100

    drm/i915: Fix use-after-free in do_switch

It then becomes apparent that the default context cannot be the context
being switched to for context switch because it is always bound. It
follows that if the ring->last_context (from) has changed after the
bind_to_gtt, it will always be the default context - this is commented
in the code block.

This assertion will help catch issues without our logic sooner than
letting the system move long (which is possible for some time).

I really want this to be a BUG(), but I also want the patch to get
merged. I think the fact that none of the ERRNOs make any sense at all
is just more evidence that this shouldn't be a WARN.

//Cc: Ian Lister (don't have current email address)
Cc: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 0e6e743..cf7cf81 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -669,6 +669,15 @@ static int do_switch_rcs(struct intel_engine_cs *ring,
 	 */
 	from = ring->last_context;
 
+	/* The only context which 'from' can be, if it was changed, is the default
+	 * context. The default context cannot end up in evict everything (as
+	 * commented above) because it is always pinned.
+	 */
+	if (WARN_ON(from == to)) {
+		ret = -EPERM;
+		goto unpin_out;
+	}
+
 	if (needs_pd_load) {
 		/* Older GENs still want the load first, "PP_DCLV followed by
 		 * PP_DIR_BASE register through Load Register Immediate commands
-- 
2.0.1