Add the drm_gem_huge_mnt_create() helper to avoid code duplication in
the i915, V3D, Panfrost and Panthor drivers. It creates and mounts a
dedicated huge tmpfs mountpoint, for the lifetime of a drm device,
used at GEM object initialization.

The next commits will port drivers to this helper.

v3:
- store huge tmpfs mountpoint in drm_device

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
---
 drivers/gpu/drm/drm_gem.c | 67 +++++++++++++++++++++++++++++++++++++++
 include/drm/drm_device.h  | 11 +++++++
 include/drm/drm_gem.h     |  1 +
 3 files changed, 79 insertions(+)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index f5a10ff363c9..b4e4947210ef 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -29,6 +29,7 @@
 #include <linux/export.h>
 #include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/fs_context.h>
 #include <linux/iosys-map.h>
 #include <linux/mem_encrypt.h>
 #include <linux/mm.h>
@@ -82,6 +83,72 @@
  * up at a later date, and as our interface with shmfs for memory allocation.
  */
 
+static int drm_gem_add_fc_param(struct fs_context *fc, const char *key,
+				const char *value)
+{
+	return vfs_parse_fs_string(fc, key, value, strlen(value));
+}
+
+static void drm_gem_huge_mnt_free(struct drm_device *dev, void *data)
+{
+	drm_WARN_ON(dev, dev->huge_mnt == NULL);
+
+	kern_unmount(dev->huge_mnt);
+	dev->huge_mnt = NULL;
+}
+
+/**
+ * drm_gem_huge_mnt_create - Create, mount and use a huge tmpfs mountpoint
+ * @dev: drm_device a huge tmpfs mountpoint should be used with
+ * @value: huge tmpfs mount option value
+ *
+ * This function creates and mounts a dedicated huge tmpfs mountpoint for the
+ * lifetime of the drm device @dev which is used at GEM object initialization
+ * with drm_gem_object_init().
+ *
+ * The most common option value @value is "within_size" which only allocates
+ * huge pages if the page will be fully within the GEM object size. "always",
+ * "advise" and "never" are supported too but the latter would just create a
+ * mountpoint similar to the default one (`shm_mnt`). See shmemfs and
+ * Transparent Hugepage for more information.
+ *
+ * Returns:
+ * 0 on success or a negative error code on failure. Returns -EEXIST if called
+ * more than once for the lifetime of the drm device.
+ */
+int drm_gem_huge_mnt_create(struct drm_device *dev, const char *value)
+{
+	struct file_system_type *type;
+	struct fs_context *fc;
+	int ret;
+
+	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
+		return -EOPNOTSUPP;
+
+	type = get_fs_type("tmpfs");
+	if (unlikely(!type))
+		return -EOPNOTSUPP;
+
+	if (unlikely(dev->huge_mnt))
+		return -EEXIST;
+
+	fc = fs_context_for_mount(type, SB_KERNMOUNT);
+	if (IS_ERR(fc))
+		return PTR_ERR(fc);
+	ret = drm_gem_add_fc_param(fc, "source", "tmpfs");
+	if (unlikely(ret))
+		return -ENOPARAM;
+	ret = drm_gem_add_fc_param(fc, "huge", value);
+	if (unlikely(ret))
+		return -ENOPARAM;
+
+	dev->huge_mnt = fc_mount_longterm(fc);
+	put_fs_context(fc);
+
+	return drmm_add_action_or_reset(dev, drm_gem_huge_mnt_free, NULL);
+}
+EXPORT_SYMBOL_GPL(drm_gem_huge_mnt_create);
+
 static void
 drm_gem_init_release(struct drm_device *dev, void *ptr)
 {
diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
index 59fd3f4d5995..895b0ced972e 100644
--- a/include/drm/drm_device.h
+++ b/include/drm/drm_device.h
@@ -3,6 +3,7 @@
 
 #include <linux/list.h>
 #include <linux/kref.h>
+#include <linux/mount.h>
 #include <linux/mutex.h>
 #include <linux/idr.h>
 #include <linux/sched.h>
@@ -168,6 +169,16 @@ struct drm_device {
 	 */
 	struct drm_master *master;
 
+	/**
+	 * @huge_mnt:
+	 *
+	 * Huge tmpfs mountpoint used at GEM object initialization
+	 * drm_gem_object_init(). Drivers can call drm_gem_huge_mnt_create() to
+	 * create a huge tmfps mountpoint. The default tmpfs mountpoint
+	 * (`shm_mnt`) is used if NULL.
+	 */
+	struct vfsmount *huge_mnt;
+
 	/**
 	 * @driver_features: per-device driver features
 	 *
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 7c8bd67d087c..7285a62d9afc 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -492,6 +492,7 @@ struct drm_gem_object {
 		DRM_GEM_FOPS,\
 	}
 
+int drm_gem_huge_mnt_create(struct drm_device *dev, const char *value);
 void drm_gem_object_release(struct drm_gem_object *obj);
 void drm_gem_object_free(struct kref *kref);
 int drm_gem_object_init(struct drm_device *dev,
-- 
2.47.3