dma-buf: Clarify that dma-buf sg lists are page aligned

The dma-buf API have been used under the assumption that the sg lists
returned from dma_buf_map_attachment() are fully page aligned. Lots of
stuff can break otherwise all over the place. Clarify this in the
documentation and add a check when DMA API debug is enabled.

Signed-off-by: Jianxin Xiong <jianxin.xiong@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1602692161-107096-1-git-send-email-jianxin.xiong@intel.com

drivers/dma-buf/dma-buf.c

@@ -849,6 +849,9 @@ EXPORT_SYMBOL_GPL(dma_buf_unpin);
  * Returns sg_table containing the scatterlist to be returned; returns ERR_PTR
  * on error. May return -EINTR if it is interrupted by a signal.
  *
+ * On success, the DMA addresses and lengths in the returned scatterlist are
+ * PAGE_SIZE aligned.
+ *
  * A mapping must be unmapped by using dma_buf_unmap_attachment(). Note that
  * the underlying backing storage is pinned for as long as a mapping exists,
  * therefore users/importers should not hold onto a mapping for undue amounts of
@@ -902,6 +905,24 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
 		attach->dir = direction;
 	}
 
+#ifdef CONFIG_DMA_API_DEBUG
+	{
+		struct scatterlist *sg;
+		u64 addr;
+		int len;
+		int i;
+
+		for_each_sgtable_dma_sg(sg_table, sg, i) {
+			addr = sg_dma_address(sg);
+			len = sg_dma_len(sg);
+			if (!PAGE_ALIGNED(addr) || !PAGE_ALIGNED(len)) {
+				pr_debug("%s: addr %llx or len %x is not page aligned!\n",
+					 __func__, addr, len);
+			}
+		}
+	}
+#endif /* CONFIG_DMA_API_DEBUG */
+
 	return sg_table;
 }
 EXPORT_SYMBOL_GPL(dma_buf_map_attachment);

include/linux/dma-buf.h

@@ -146,7 +146,8 @@ struct dma_buf_ops {
 	 *
 	 * A &sg_table scatter list of or the backing storage of the DMA buffer,
 	 * already mapped into the device address space of the &device attached
-	 * with the provided &dma_buf_attachment.
+	 * with the provided &dma_buf_attachment. The addresses and lengths in
+	 * the scatter list are PAGE_SIZE aligned.
 	 *
 	 * On failure, returns a negative error value wrapped into a pointer.
 	 * May also return -EINTR when a signal was received while being