drm/ttm: add TTM_TT_FLAG_EXTERNAL_MAPPABLE

In commit:

commit 667a50d
Author: Thomas Hellstrom <thellstrom@vmware.com>
Date:   Fri Jan 3 11:17:18 2014 +0100

    drm/ttm: Refuse to fault (prime-) imported pages

we introduced the restriction that imported pages should not be directly
mappable through TTM(this also extends to userptr). In the next patch we
want to introduce a shmem_tt backend, which should follow all the
existing rules with TTM_PAGE_FLAG_EXTERNAL, since it will need to handle
swapping itself, but with the above mapping restriction lifted.

v2(Christian):
  - Don't OR together EXTERNAL and EXTERNAL_MAPPABLE in the definition
    of EXTERNAL_MAPPABLE, just leave it the caller to handle this
    correctly, otherwise we might encounter subtle issues.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929132629.353541-3-matthew.auld@intel.com
Signed-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/ttm/ttm_bo_vm.c

@@ -163,8 +163,10 @@ vm_fault_t ttm_bo_vm_reserve(struct ttm_buffer_object *bo,
 	 * (if at all) by redirecting mmap to the exporter.
 	 */
 	if (bo->ttm && (bo->ttm->page_flags & TTM_TT_FLAG_EXTERNAL)) {
-		dma_resv_unlock(bo->base.resv);
-		return VM_FAULT_SIGBUS;
+		if (!(bo->ttm->page_flags & TTM_TT_FLAG_EXTERNAL_MAPPABLE)) {
+			dma_resv_unlock(bo->base.resv);
+			return VM_FAULT_SIGBUS;
+		}
 	}
 
 	return 0;

drivers/gpu/drm/ttm/ttm_tt.c

@@ -84,6 +84,9 @@ int ttm_tt_create(struct ttm_buffer_object *bo, bool zero_alloc)
 	if (unlikely(bo->ttm == NULL))
 		return -ENOMEM;
 
+	WARN_ON(bo->ttm->page_flags & TTM_TT_FLAG_EXTERNAL_MAPPABLE &&
+		!(bo->ttm->page_flags & TTM_TT_FLAG_EXTERNAL));
+
 	return 0;
 }
 

include/drm/ttm/ttm_tt.h

@@ -67,13 +67,26 @@ struct ttm_tt {
 	 * Note that enum ttm_bo_type.ttm_bo_type_sg objects will always enable
 	 * this flag.
 	 *
+	 * TTM_TT_FLAG_EXTERNAL_MAPPABLE: Same behaviour as
+	 * TTM_TT_FLAG_EXTERNAL, but with the reduced restriction that it is
+	 * still valid to use TTM to map the pages directly. This is useful when
+	 * implementing a ttm_tt backend which still allocates driver owned
+	 * pages underneath(say with shmem).
+	 *
+	 * Note that since this also implies TTM_TT_FLAG_EXTERNAL, the usage
+	 * here should always be:
+	 *
+	 *   page_flags = TTM_TT_FLAG_EXTERNAL |
+	 *		  TTM_TT_FLAG_EXTERNAL_MAPPABLE;
+	 *
 	 * TTM_TT_FLAG_PRIV_POPULATED: TTM internal only. DO NOT USE. This is
 	 * set by TTM after ttm_tt_populate() has successfully returned, and is
 	 * then unset when TTM calls ttm_tt_unpopulate().
 	 */
-#define TTM_TT_FLAG_SWAPPED	(1 << 0)
-#define TTM_TT_FLAG_ZERO_ALLOC	(1 << 1)
-#define TTM_TT_FLAG_EXTERNAL	(1 << 2)
+#define TTM_TT_FLAG_SWAPPED		(1 << 0)
+#define TTM_TT_FLAG_ZERO_ALLOC		(1 << 1)
+#define TTM_TT_FLAG_EXTERNAL		(1 << 2)
+#define TTM_TT_FLAG_EXTERNAL_MAPPABLE	(1 << 3)
 
 #define TTM_TT_FLAG_PRIV_POPULATED  (1 << 31)
 	uint32_t page_flags;