drm: Move legacy device list out of drm_driver

The drm_driver structure contains a single field (legacy_dev_list) that
is modified by the DRM core, used to store a linked list of legacy DRM
devices associated with the driver. In order to make the structure
const, move the field out to a global variable. This requires locking
access to the global where the local field didn't require serialization,
but this only affects legacy drivers, and isn't in any hot path.

While at it, compile-out the legacy_dev_list field when DRM_LEGACY isn't
defined.

Signed-off-by: Laurent Pinchart <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Reviewed-by: Emil Velikov <[e-mail]>

drivers/gpu/drm/drm_pci.c

···
 
 #include <linux/dma-mapping.h>
 #include <linux/export.h>
+#include <linux/list.h>
+#include <linux/mutex.h>
 #include <linux/pci.h>
 #include <linux/slab.h>
 
···
 #include "drm_legacy.h"
 
 #ifdef CONFIG_DRM_LEGACY
+/* List of devices hanging off drivers with stealth attach. */
+static LIST_HEAD(legacy_dev_list);
+static DEFINE_MUTEX(legacy_dev_list_lock);
 
 /**
  * drm_pci_alloc - Allocate a PCI consistent memory block, for DMA.
···
 	if (ret)
 		goto err_agp;
 
-	/* No locking needed since shadow-attach is single-threaded since it may
-	 * only be called from the per-driver module init hook. */
-	if (drm_core_check_feature(dev, DRIVER_LEGACY))
-		list_add_tail(&dev->legacy_dev_list, &driver->legacy_dev_list);
+	if (drm_core_check_feature(dev, DRIVER_LEGACY)) {
+		mutex_lock(&legacy_dev_list_lock);
+		list_add_tail(&dev->legacy_dev_list, &legacy_dev_list);
+		mutex_unlock(&legacy_dev_list_lock);
+	}
 
 	return 0;
 
···
 		return -EINVAL;
 
 	/* If not using KMS, fall back to stealth mode manual scanning. */
-	INIT_LIST_HEAD(&driver->legacy_dev_list);
 	for (i = 0; pdriver->id_table[i].vendor != 0; i++) {
 		pid = &pdriver->id_table[i];
 
···
 	if (!(driver->driver_features & DRIVER_LEGACY)) {
 		WARN_ON(1);
 	} else {
-		list_for_each_entry_safe(dev, tmp, &driver->legacy_dev_list,
+		mutex_lock(&legacy_dev_list_lock);
+		list_for_each_entry_safe(dev, tmp, &legacy_dev_list,
 					 legacy_dev_list) {
-			list_del(&dev->legacy_dev_list);
-			drm_put_dev(dev);
+			if (dev->driver == driver) {
+				list_del(&dev->legacy_dev_list);
+				drm_put_dev(dev);
+			}
 		}
+		mutex_unlock(&legacy_dev_list_lock);
 	}
 	DRM_INFO("Module unloaded\n");
 }

include/drm/drm_device.h

···
  * may contain multiple heads.
  */
 struct drm_device {
-	/**
-	 * @legacy_dev_list:
-	 *
-	 * List of devices per driver for stealth attach cleanup
-	 */
-	struct list_head legacy_dev_list;
-
 	/** @if_version: Highest interface version set */
 	int if_version;
 
···
 	/* Everything below here is for legacy driver, never use! */
 	/* private: */
 #if IS_ENABLED(CONFIG_DRM_LEGACY)
+	/* List of devices per driver for stealth attach cleanup */
+	struct list_head legacy_dev_list;
+
 	/* Context handle management - linked list of context handles */
 	struct list_head ctxlist;
 

include/drm/drm_drv.h

···
 	/* Everything below here is for legacy driver, never use! */
 	/* private: */
 
-	/* List of devices hanging off this driver with stealth attach. */
-	struct list_head legacy_dev_list;
 	int (*firstopen) (struct drm_device *);
 	void (*preclose) (struct drm_device *, struct drm_file *file_priv);
 	int (*dma_ioctl) (struct drm_device *dev, void *data, struct drm_file *file_priv);