drm/xe/hw_engine_group: Ensure safe transition between execution modes

Provide a way to safely transition execution modes of the hw engine group
ahead of the actual execution. When necessary, either wait for running
jobs to complete or preempt them, thus ensuring mutual exclusion between
execution modes.

Unlike a mutex, the rw_semaphore used in this context allows multiple
submissions in the same mode.

v2: Use lockdep_assert_held_write, add annotations (Matt Brost)

v3: Fix kernel doc, remove redundant code (Matt Brost)

v4: Now that xe_hw_engine_group_suspend_faulting_lr_jobs can fail,
    propagate the error to the caller (Matt Brost)
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240809155156.1955925-9-francois.dugast@intel.com

drivers/gpu/drm/xe/xe_hw_engine_group.c

@@ -235,3 +235,78 @@ static int xe_hw_engine_group_wait_for_dma_fence_jobs(struct xe_hw_engine_group
 
 	return 0;
 }
+
+static int switch_mode(struct xe_hw_engine_group *group)
+{
+	int err = 0;
+	enum xe_hw_engine_group_execution_mode new_mode;
+
+	lockdep_assert_held_write(&group->mode_sem);
+
+	switch (group->cur_mode) {
+	case EXEC_MODE_LR:
+		new_mode = EXEC_MODE_DMA_FENCE;
+		err = xe_hw_engine_group_suspend_faulting_lr_jobs(group);
+		break;
+	case EXEC_MODE_DMA_FENCE:
+		new_mode = EXEC_MODE_LR;
+		err = xe_hw_engine_group_wait_for_dma_fence_jobs(group);
+		break;
+	}
+
+	if (err)
+		return err;
+
+	group->cur_mode = new_mode;
+
+	return 0;
+}
+
+/**
+ * xe_hw_engine_group_get_mode() - Get the group to execute in the new mode
+ * @group: The hw engine group
+ * @new_mode: The new execution mode
+ * @previous_mode: Pointer to the previous mode provided for use by caller
+ *
+ * Return: 0 if successful, -EINTR if locking failed.
+ */
+int xe_hw_engine_group_get_mode(struct xe_hw_engine_group *group,
+				enum xe_hw_engine_group_execution_mode new_mode,
+				enum xe_hw_engine_group_execution_mode *previous_mode)
+__acquires(&group->mode_sem)
+{
+	int err = down_read_interruptible(&group->mode_sem);
+
+	if (err)
+		return err;
+
+	*previous_mode = group->cur_mode;
+
+	if (new_mode != group->cur_mode) {
+		up_read(&group->mode_sem);
+		err = down_write_killable(&group->mode_sem);
+		if (err)
+			return err;
+
+		if (new_mode != group->cur_mode) {
+			err = switch_mode(group);
+			if (err) {
+				up_write(&group->mode_sem);
+				return err;
+			}
+		}
+		downgrade_write(&group->mode_sem);
+	}
+
+	return err;
+}
+
+/**
+ * xe_hw_engine_group_put() - Put the group
+ * @group: The hw engine group
+ */
+void xe_hw_engine_group_put(struct xe_hw_engine_group *group)
+__releases(&group->mode_sem)
+{
+	up_read(&group->mode_sem);
+}

drivers/gpu/drm/xe/xe_hw_engine_group.h

@@ -17,4 +17,9 @@ int xe_hw_engine_setup_groups(struct xe_gt *gt);
 int xe_hw_engine_group_add_exec_queue(struct xe_hw_engine_group *group, struct xe_exec_queue *q);
 void xe_hw_engine_group_del_exec_queue(struct xe_hw_engine_group *group, struct xe_exec_queue *q);
 
+int xe_hw_engine_group_get_mode(struct xe_hw_engine_group *group,
+				enum xe_hw_engine_group_execution_mode new_mode,
+				enum xe_hw_engine_group_execution_mode *previous_mode);
+void xe_hw_engine_group_put(struct xe_hw_engine_group *group);
+
 #endif