drm/i915: Use locked access to ctx->engines in set_priority

This essentially reverts

commit 89ff76b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Apr 2 13:42:18 2020 +0100

    drm/i915/gem: Utilize rcu iteration of context engines

Note that the other use of __context_engines_await have disappeard in
the following commits:

ccbc1b9 ("drm/i915/gem: Don't allow changing the VM on running contexts (v4)")
c7a71fc ("drm/i915: Drop getparam support for I915_CONTEXT_PARAM_ENGINES")
4a766ae ("drm/i915: Drop the CONTEXT_CLONE API (v2)")

None of these have any business to optimize their engine lookup with
rcu, unless extremely convincing benchmark data and a solid analysis
why we can't make that workload (whatever it is that does) faster with
a proper design fix.

Also since there's only one caller of context_apply_all left and it's
really just a loop, inline it and then inline the lopp body too. This
is how all other callers that take the engine lock loop over engines,
it's much simpler.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210810130523.1972031-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -1284,49 +1284,6 @@ static int __context_set_persistence(struct i915_gem_context *ctx, bool state)
 	return 0;
 }
 
-static inline struct i915_gem_engines *
-__context_engines_await(const struct i915_gem_context *ctx,
-			bool *user_engines)
-{
-	struct i915_gem_engines *engines;
-
-	rcu_read_lock();
-	do {
-		engines = rcu_dereference(ctx->engines);
-		GEM_BUG_ON(!engines);
-
-		if (user_engines)
-			*user_engines = i915_gem_context_user_engines(ctx);
-
-		/* successful await => strong mb */
-		if (unlikely(!i915_sw_fence_await(&engines->fence)))
-			continue;
-
-		if (likely(engines == rcu_access_pointer(ctx->engines)))
-			break;
-
-		i915_sw_fence_complete(&engines->fence);
-	} while (1);
-	rcu_read_unlock();
-
-	return engines;
-}
-
-static void
-context_apply_all(struct i915_gem_context *ctx,
-		  void (*fn)(struct intel_context *ce, void *data),
-		  void *data)
-{
-	struct i915_gem_engines_iter it;
-	struct i915_gem_engines *e;
-	struct intel_context *ce;
-
-	e = __context_engines_await(ctx, NULL);
-	for_each_gem_engine(ce, e, it)
-		fn(ce, data);
-	i915_sw_fence_complete(&e->fence);
-}
-
 static struct i915_gem_context *
 i915_gem_create_context(struct drm_i915_private *i915,
 			const struct i915_gem_proto_context *pc)
@@ -1776,31 +1733,30 @@ set_persistence(struct i915_gem_context *ctx,
 	return __context_set_persistence(ctx, args->value);
 }
 
-static void __apply_priority(struct intel_context *ce, void *arg)
-{
-	struct i915_gem_context *ctx = arg;
-
-	if (!intel_engine_has_timeslices(ce->engine))
-		return;
-
-	if (ctx->sched.priority >= I915_PRIORITY_NORMAL &&
-	    intel_engine_has_semaphores(ce->engine))
-		intel_context_set_use_semaphores(ce);
-	else
-		intel_context_clear_use_semaphores(ce);
-}
-
 static int set_priority(struct i915_gem_context *ctx,
 			const struct drm_i915_gem_context_param *args)
 {
+	struct i915_gem_engines_iter it;
+	struct intel_context *ce;
 	int err;
 
 	err = validate_priority(ctx->i915, args);
 	if (err)
 		return err;
 
 	ctx->sched.priority = args->value;
-	context_apply_all(ctx, __apply_priority, ctx);
+
+	for_each_gem_engine(ce, i915_gem_context_lock_engines(ctx), it) {
+		if (!intel_engine_has_timeslices(ce->engine))
+			continue;
+
+		if (ctx->sched.priority >= I915_PRIORITY_NORMAL &&
+		    intel_engine_has_semaphores(ce->engine))
+			intel_context_set_use_semaphores(ce);
+		else
+			intel_context_clear_use_semaphores(ce);
+	}
+	i915_gem_context_unlock_engines(ctx);
 
 	return 0;
 }