[RFC PATCH v3 14/21] drm/exec: Introduce a drm_exec_trylock_obj() function

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

TTM needs to trylock dma_resv objects in a couple of places. However
this functionality is missing in drm_exec. Introduce it.

Note that in addition to the -EBUSY error returned on failure to
take the lock, the operation may return -ENOMEM if there was a
failure to allocate memory for the drm_exec held lock array.

This failure mode could be avoided if the drm_exec structure instead
maintained a linked list of locked objects, similar to i915.

Cc: Christian König <christian.koenig@amd.com>
Cc: Somalapuram Amaranath <Amaranath.Somalapuram@amd.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: <dri-devel@lists.freedesktop.org>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 drivers/gpu/drm/drm_exec.c | 50 +++++++++++++++++++++++++++++++++++---
 include/drm/drm_exec.h     |  1 +
 2 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
index 3770a5d30213..1383680ffa4a 100644
--- a/drivers/gpu/drm/drm_exec.c
+++ b/drivers/gpu/drm/drm_exec.c
@@ -139,14 +139,17 @@ EXPORT_SYMBOL(drm_exec_cleanup);
 
 /* Track the locked object in the array */
 static int drm_exec_obj_locked(struct drm_exec *exec,
-			       struct drm_gem_object *obj)
+			       struct drm_gem_object *obj,
+			       gfp_t gfp)
 {
+	might_alloc(gfp);
+
 	if (unlikely(exec->num_objects == exec->max_objects)) {
 		size_t size = exec->max_objects * sizeof(void *);
 		void *tmp;
 
 		tmp = kvrealloc(exec->objects, size, size + PAGE_SIZE,
-				GFP_KERNEL);
+				gfp);
 		if (!tmp)
 			return -ENOMEM;
 
@@ -179,7 +182,7 @@ static int drm_exec_lock_contended(struct drm_exec *exec)
 		dma_resv_lock_slow(obj->resv, &exec->ticket);
 	}
 
-	ret = drm_exec_obj_locked(exec, obj);
+	ret = drm_exec_obj_locked(exec, obj, GFP_KERNEL);
 	if (unlikely(ret))
 		goto error_unlock;
 
@@ -215,6 +218,45 @@ int drm_exec_handle_contended(struct drm_exec *exec)
 }
 EXPORT_SYMBOL(drm_exec_handle_contended);
 
+/**
+ * drm_exec_trylock_obj - trylock a GEM object for use
+ * @exec: the drm_exec object with the state.
+ * @obj: the GEM object to lock.
+ *
+ * Trylock a GEM object for use and grab a reference to it.
+ *
+ * Returns: -EALREADY when object is already locked (can be suppressed by
+ * setting the DRM_EXEC_IGNORE_DUPLICATES flag), -ENOMEM when memory
+ * allocation failed, and zero for success. If the object was already
+ * locked, -EBUSY will be returned.
+ */
+int drm_exec_trylock_obj(struct drm_exec *exec, struct drm_gem_object *obj)
+{
+	int ret;
+
+	might_alloc(GFP_ATOMIC);
+
+	if (exec->prelocked == obj) {
+		drm_gem_object_put(exec->prelocked);
+		exec->prelocked = NULL;
+		return 0;
+	}
+
+	if (!dma_resv_trylock_ctx(obj->resv, &exec->ticket)) {
+		if (dma_resv_locking_ctx(obj->resv) == &exec->ticket)
+			return (exec->flags & DRM_EXEC_IGNORE_DUPLICATES) ? 0 : -EALREADY;
+		else
+			return -EBUSY;
+	}
+
+	ret = drm_exec_obj_locked(exec, obj, GFP_ATOMIC | __GFP_NOWARN);
+	if (ret)
+		dma_resv_unlock(obj->resv);
+
+	return ret;
+}
+EXPORT_SYMBOL(drm_exec_trylock_obj);
+
 /**
  * drm_exec_lock_obj - lock a GEM object for use
  * @exec: the drm_exec object with the state
@@ -254,7 +296,7 @@ int drm_exec_lock_obj(struct drm_exec *exec, struct drm_gem_object *obj)
 	if (unlikely(ret))
 		return ret;
 
-	ret = drm_exec_obj_locked(exec, obj);
+	ret = drm_exec_obj_locked(exec, obj, GFP_KERNEL);
 	if (ret)
 		goto error_unlock;
 
diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
index fafb40d96e38..ea0f2117ee0c 100644
--- a/include/drm/drm_exec.h
+++ b/include/drm/drm_exec.h
@@ -152,6 +152,7 @@ void drm_exec_init(struct drm_exec *exec, u32 flags, unsigned nr);
 void drm_exec_fini(struct drm_exec *exec);
 bool drm_exec_cleanup(struct drm_exec *exec);
 int drm_exec_lock_obj(struct drm_exec *exec, struct drm_gem_object *obj);
+int drm_exec_trylock_obj(struct drm_exec *exec, struct drm_gem_object *obj);
 void drm_exec_unlock_obj(struct drm_exec *exec, struct drm_gem_object *obj);
 int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
 			 unsigned int num_fences);
-- 
2.44.0