[PATCH 4/6] drm/ttm: Support kmap for single-page mappings in ttm_bo_vmap()

[Thomas Zimmermann <tzimmermann@suse.de>]

In ttm_bo_vmap(), set up single-page mappings with kmap() in certain
cases. The feature is already present in ttm_bo_kmap().

This functionality is require by DRM's xe driver, which claims that
using kmap() is an optimization over vmap(). [1] Reading the commit
at [2] indicates otherwise. It is not possible to use kmap_local_page()
and kunmap_local_page(), as TTM cannot guarantee the requirements for
ordering these calls. [3]

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://elixir.bootlin.com/linux/v6.9/source/drivers/gpu/drm/xe/xe_bo.c#L1870 # 1
Link: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com/T/#u # 2
Link: https://elixir.bootlin.com/linux/v6.9/source/include/linux/highmem.h#L70 # 3
---
 drivers/gpu/drm/ttm/ttm_bo_util.c | 33 ++++++++++++++++++++++---------
 1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index 31f9772f05dac..c06cfccace39d 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -516,6 +516,8 @@ int ttm_bo_vmap(struct ttm_buffer_object *bo,
 			.no_wait_gpu = false
 		};
 		struct ttm_tt *ttm = bo->ttm;
+		struct ttm_resource_manager *man =
+			ttm_manager_type(bo->bdev, bo->resource->mem_type);
 		unsigned long start_page = offset >> PAGE_SHIFT;
 		unsigned long aligned_size = size + (offset - (start_page << PAGE_SHIFT));
 		unsigned long num_pages = DIV_ROUND_UP(aligned_size, PAGE_SIZE);
@@ -527,15 +529,25 @@ int ttm_bo_vmap(struct ttm_buffer_object *bo,
 		if (ret)
 			return ret;
 
-		/*
-		 * We need to use vmap to get the desired page protection
-		 * or to make the buffer object look contiguous.
-		 */
-		prot = ttm_io_prot(bo, mem, PAGE_KERNEL);
-		vaddr = vmap(ttm->pages + start_page, num_pages, 0, prot);
-		if (!vaddr)
-			return -ENOMEM;
-		alloc_flags = ttm_bo_map_vmap;
+		if (num_pages == 1 && ttm->caching == ttm_cached &&
+		    !(man->use_tt && (ttm->page_flags & TTM_TT_FLAG_DECRYPTED))) {
+			/*
+			 * We're mapping a single page, and the desired
+			 * page protection is consistent with the bo.
+			 */
+			vaddr = kmap(ttm->pages[start_page]);
+			alloc_flags = ttm_bo_map_kmap;
+		} else {
+			/*
+			 * We need to use vmap to get the desired page protection
+			 * or to make the buffer object look contiguous.
+			 */
+			prot = ttm_io_prot(bo, mem, PAGE_KERNEL);
+			vaddr = vmap(ttm->pages + start_page, num_pages, 0, prot);
+			if (!vaddr)
+				return -ENOMEM;
+			alloc_flags = ttm_bo_map_vmap;
+		}
 
 		iosys_map_set_vaddr(map, vaddr);
 		map->alloc_flags = alloc_flags;
@@ -567,6 +579,9 @@ void ttm_bo_vunmap(struct ttm_buffer_object *bo, struct iosys_map *map)
 	case ttm_bo_map_vmap:
 		vunmap(map->vaddr);
 		break;
+	case ttm_bo_map_kmap:
+		kunmap(kmap_to_page(map->vaddr));
+		break;
 	case ttm_bo_map_premapped:
 		break;
 	default:
-- 
2.45.2