[PATCH v4 04/38] drm: prime: add common helper to check scatterlist contiguity

[Marek Szyprowski]

It is a common operation done by DRM drivers to check the contiguity
of the DMA-mapped buffer described by a scatterlist in the
sg_table object. Let's add a common helper for this operation.

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
For more information, see '[PATCH v4 00/38] DRM: fix struct sg_table nents
vs. orig_nents misuse' thread:
https://lore.kernel.org/dri-devel/20200512085710.14688-1-m.szyprowski@xxxxxxxxxxx/T/
---
 drivers/gpu/drm/drm_gem_cma_helper.c | 23 +++--------------------
 drivers/gpu/drm/drm_prime.c          | 26 ++++++++++++++++++++++++++
 include/drm/drm_prime.h              |  2 ++
 3 files changed, 31 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
index 12e98fb..9f2d13e 100644
--- a/drivers/gpu/drm/drm_gem_cma_helper.c
+++ b/drivers/gpu/drm/drm_gem_cma_helper.c
@@ -471,26 +471,9 @@ struct drm_gem_object *
 {
 	struct drm_gem_cma_object *cma_obj;
 
-	if (sgt->nents != 1) {
-		/* check if the entries in the sg_table are contiguous */
-		dma_addr_t next_addr = sg_dma_address(sgt->sgl);
-		struct scatterlist *s;
-		unsigned int i;
-
-		for_each_sg(sgt->sgl, s, sgt->nents, i) {
-			/*
-			 * sg_dma_address(s) is only valid for entries
-			 * that have sg_dma_len(s) != 0
-			 */
-			if (!sg_dma_len(s))
-				continue;
-
-			if (sg_dma_address(s) != next_addr)
-				return ERR_PTR(-EINVAL);
-
-			next_addr = sg_dma_address(s) + sg_dma_len(s);
-		}
-	}
+	/* check if the entries in the sg_table are contiguous */
+	if (drm_prime_get_contiguous_size(sgt) < attach->dmabuf->size)
+		return ERR_PTR(-EINVAL);
 
 	/* Create a CMA GEM buffer. */
 	cma_obj = __drm_gem_cma_create(dev, attach->dmabuf->size);
diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 282774e..1d2e5fe 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -826,6 +826,32 @@ struct sg_table *drm_prime_pages_to_sg(struct page **pages, unsigned int nr_page
 EXPORT_SYMBOL(drm_prime_pages_to_sg);
 
 /**
+ * drm_prime_get_contiguous_size - returns the contiguous size of the buffer
+ * @sgt: sg_table describing the buffer to check
+ *
+ * This helper calculates the contiguous size in the DMA address space
+ * of the the buffer described by the provided sg_table.
+ *
+ * This is useful for implementing
+ * &drm_gem_object_funcs.gem_prime_import_sg_table.
+ */
+unsigned long drm_prime_get_contiguous_size(struct sg_table *sgt)
+{
+	dma_addr_t expected = sg_dma_address(sgt->sgl);
+	struct sg_dma_page_iter dma_iter;
+	unsigned long size = 0;
+
+	for_each_sgtable_dma_page(sgt, &dma_iter, 0) {
+		if (sg_page_iter_dma_address(&dma_iter) != expected)
+			break;
+		expected += PAGE_SIZE;
+		size += PAGE_SIZE;
+	}
+	return size;
+}
+EXPORT_SYMBOL(drm_prime_get_contiguous_size);
+
+/**
  * drm_gem_prime_export - helper library implementation of the export callback
  * @obj: GEM object to export
  * @flags: flags like DRM_CLOEXEC and DRM_RDWR
diff --git a/include/drm/drm_prime.h b/include/drm/drm_prime.h
index 9af7422..47ef116 100644
--- a/include/drm/drm_prime.h
+++ b/include/drm/drm_prime.h
@@ -92,6 +92,8 @@ void drm_gem_unmap_dma_buf(struct dma_buf_attachment *attach,
 struct dma_buf *drm_gem_prime_export(struct drm_gem_object *obj,
 				     int flags);
 
+unsigned long drm_prime_get_contiguous_size(struct sg_table *sgt);
+
 /* helper functions for importing */
 struct drm_gem_object *drm_gem_prime_import_dev(struct drm_device *dev,
 						struct dma_buf *dma_buf,
-- 
1.9.1