[RFC PATCH v2 2/6] drm/shmem: Introduce the notion of sparse objects

["Adrián Larumbe" <adrian.larumbe@collabora.com>]

Sparse DRM objects will store their backing pages in an xarray, to avoid
the overhead of preallocating a huge struct page pointer array when only
a very small range of indices might be assigned.

For now, only the definition of a sparse object as a union alternative
to a 'dense' object is provided, with functions that exploit it being
part of later commits.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 68 +++++++++++++++++++++++++-
 include/drm/drm_gem_shmem_helper.h     | 23 ++++++++-
 2 files changed, 88 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index d99dee67353a..5f75eb1230f6 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -128,6 +128,31 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_create);
 
+/**
+ * drm_gem_shmem_create_sparse - Allocate a sparse object with the given size
+ * @dev: DRM device
+ * @size: Size of the sparse object to allocate
+ *
+ * This function creates a sparse shmem GEM object.
+ *
+ * Returns:
+ * A struct drm_gem_shmem_object * on success or an ERR_PTR()-encoded negative
+ * error code on failure.
+ */
+struct drm_gem_shmem_object *drm_gem_shmem_create_sparse(struct drm_device *dev, size_t size)
+{
+	struct drm_gem_shmem_object *shmem =
+		__drm_gem_shmem_create(dev, size, false, NULL);
+
+	if (!IS_ERR(shmem)) {
+		shmem->sparse = true;
+		xa_init_flags(&shmem->xapages, XA_FLAGS_ALLOC);
+	}
+
+	return shmem;
+}
+EXPORT_SYMBOL_GPL(drm_gem_shmem_create_sparse);
+
 /**
  * drm_gem_shmem_create_with_mnt - Allocate an object with the given size in a
  * given mountpoint
@@ -173,8 +198,8 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 			sg_free_table(shmem->sgt);
 			kfree(shmem->sgt);
 		}
-		if (shmem->pages)
-			drm_gem_shmem_put_pages(shmem);
+
+		drm_gem_shmem_put_pages(shmem);
 
 		drm_WARN_ON(obj->dev, shmem->pages_use_count);
 
@@ -196,6 +221,12 @@ static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
 	if (shmem->pages_use_count++ > 0)
 		return 0;
 
+	/* We only allow increasing the user count in the case of
+	 * sparse shmem objects with some backed pages for now
+	 */
+	if (shmem->sparse && xa_empty(&shmem->xapages))
+		return -EINVAL;
+
 	pages = drm_gem_get_pages(obj);
 	if (IS_ERR(pages)) {
 		drm_dbg_kms(obj->dev, "Failed to get pages (%ld)\n",
@@ -231,6 +262,14 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
 
 	dma_resv_assert_held(shmem->base.resv);
 
+	if (!shmem->sparse) {
+		if (!shmem->pages)
+			return;
+	} else {
+		/* Not implemented yet */
+		return;
+	}
+
 	if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
 		return;
 
@@ -404,8 +443,15 @@ void drm_gem_shmem_vunmap(struct drm_gem_shmem_object *shmem,
 {
 	struct drm_gem_object *obj = &shmem->base;
 
+	if (shmem->sparse) {
+		drm_err(obj->dev, "UM unmapping of sparse shmem objects not implemented\n");
+		return;
+	}
+
 	if (drm_gem_is_imported(obj)) {
 		dma_buf_vunmap(obj->dma_buf, map);
+	} else if (obj->import_attach) {
+		dma_buf_vunmap(obj->import_attach->dmabuf, map);
 	} else {
 		dma_resv_assert_held(shmem->base.resv);
 
@@ -541,6 +587,12 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
 	struct page *page;
 	pgoff_t page_offset;
 
+	/* TODO: Implement UM mapping of sparse shmem objects */
+	if (drm_WARN_ON(obj->dev, shmem->sparse)) {
+		drm_err(obj->dev, "UM mapping of sparse shmem objects not implemented\n");
+		return VM_FAULT_SIGBUS;
+	}
+
 	/* We don't use vmf->pgoff since that has the fake offset */
 	page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
 
@@ -566,8 +618,14 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
 	struct drm_gem_object *obj = vma->vm_private_data;
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 
+	/* TODO: Implement UM mapping of sparse shmem objects */
+	if (drm_WARN_ON(obj->dev, shmem->sparse))
+		return;
+
 	drm_WARN_ON(obj->dev, drm_gem_is_imported(obj));
 
+	drm_WARN_ON(obj->dev, obj->import_attach);
+
 	dma_resv_lock(shmem->base.resv, NULL);
 
 	/*
@@ -690,6 +748,9 @@ struct sg_table *drm_gem_shmem_get_sg_table(struct drm_gem_shmem_object *shmem)
 {
 	struct drm_gem_object *obj = &shmem->base;
 
+	if (drm_WARN_ON(obj->dev, shmem->sparse))
+		return ERR_PTR(-EINVAL);
+
 	drm_WARN_ON(obj->dev, drm_gem_is_imported(obj));
 
 	return drm_prime_pages_to_sg(obj->dev, shmem->pages, obj->size >> PAGE_SHIFT);
@@ -702,6 +763,9 @@ static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
 	int ret;
 	struct sg_table *sgt;
 
+	if (drm_WARN_ON(obj->dev, shmem->sparse))
+		return ERR_PTR(-EINVAL);
+
 	if (shmem->sgt)
 		return shmem->sgt;
 
diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
index cef5a6b5a4d6..00e47512b30f 100644
--- a/include/drm/drm_gem_shmem_helper.h
+++ b/include/drm/drm_gem_shmem_helper.h
@@ -6,6 +6,7 @@
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/mutex.h>
+#include <linux/xarray.h>
 
 #include <drm/drm_file.h>
 #include <drm/drm_gem.h>
@@ -29,7 +30,10 @@ struct drm_gem_shmem_object {
 	/**
 	 * @pages: Page table
 	 */
-	struct page **pages;
+	union {
+		struct page **pages;
+		struct xarray xapages;
+	};
 
 	/**
 	 * @pages_use_count:
@@ -91,12 +95,18 @@ struct drm_gem_shmem_object {
 	 * @map_wc: map object write-combined (instead of using shmem defaults).
 	 */
 	bool map_wc : 1;
+
+	/**
+	 * @sparse: the object is only partially backed by pages
+	 */
+	bool sparse : 1;
 };
 
 #define to_drm_gem_shmem_obj(obj) \
 	container_of(obj, struct drm_gem_shmem_object, base)
 
 struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t size);
+struct drm_gem_shmem_object *drm_gem_shmem_create_sparse(struct drm_device *dev, size_t size);
 struct drm_gem_shmem_object *drm_gem_shmem_create_with_mnt(struct drm_device *dev,
 							   size_t size,
 							   struct vfsmount *gemfs);
@@ -210,6 +220,10 @@ static inline struct sg_table *drm_gem_shmem_object_get_sg_table(struct drm_gem_
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 
+	/* Use the specific sparse shmem get_sg_table function instead */
+	if (WARN_ON(shmem->sparse))
+		return ERR_PTR(-EINVAL);
+
 	return drm_gem_shmem_get_sg_table(shmem);
 }
 
@@ -229,6 +243,10 @@ static inline int drm_gem_shmem_object_vmap(struct drm_gem_object *obj,
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 
+	/* TODO: Implement kernel mapping of sparse shmem objects */
+	if (WARN_ON(shmem->sparse))
+		return -EACCES;
+
 	return drm_gem_shmem_vmap(shmem, map);
 }
 
@@ -263,6 +281,9 @@ static inline int drm_gem_shmem_object_mmap(struct drm_gem_object *obj, struct v
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 
+	if (shmem->sparse)
+		return -EACCES;
+
 	return drm_gem_shmem_mmap(shmem, vma);
 }
 
-- 
2.48.1