[PATCH v8] rapidio: fix common struct sg_table related issues

[Marek Szyprowski]

The Documentation/DMA-API-HOWTO.txt states that the dma_map_sg() function
returns the number of the created entries in the DMA address space.
However the subsequent calls to the dma_sync_sg_for_{device,cpu}() and
dma_unmap_sg must be called with the original number of the entries
passed to the dma_map_sg().

struct sg_table is a common structure used for describing a non-contiguous
memory buffer, used commonly in the DRM and graphics subsystems. It
consists of a scatterlist with memory pages and DMA addresses (sgl entry),
as well as the number of scatterlist entries: CPU pages (orig_nents entry)
and DMA mapped pages (nents entry).

It turned out that it was a common mistake to misuse nents and orig_nents
entries, calling DMA-mapping functions with a wrong number of entries or
ignoring the number of mapped entries returned by the dma_map_sg()
function.

To avoid such issues, lets use a common dma-mapping wrappers operating
directly on the struct sg_table objects and use scatterlist page
iterators where possible. This, almost always, hides references to the
nents and orig_nents entries, making the code robust, easier to follow
and copy/paste safe.

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
v8: fixed issues pointed by kbuilt test robot (use of uninitialized
    variable)
---
 drivers/rapidio/devices/rio_mport_cdev.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/rapidio/devices/rio_mport_cdev.c b/drivers/rapidio/devices/rio_mport_cdev.c
index 3abbba8c2b5b..351c08b886ec 100644
--- a/drivers/rapidio/devices/rio_mport_cdev.c
+++ b/drivers/rapidio/devices/rio_mport_cdev.c
@@ -573,8 +573,7 @@ static void dma_req_free(struct kref *ref)
 			refcount);
 	struct mport_cdev_priv *priv = req->priv;
 
-	dma_unmap_sg(req->dmach->device->dev,
-		     req->sgt.sgl, req->sgt.nents, req->dir);
+	dma_unmap_sgtable(req->dmach->device->dev, &req->sgt, req->dir, 0);
 	sg_free_table(&req->sgt);
 	if (req->page_list) {
 		unpin_user_pages(req->page_list, req->nr_pages);
@@ -814,7 +813,6 @@ rio_dma_transfer(struct file *filp, u32 transfer_mode,
 	struct mport_dev *md = priv->md;
 	struct dma_chan *chan;
 	int ret;
-	int nents;
 
 	if (xfer->length == 0)
 		return -EINVAL;
@@ -930,15 +928,14 @@ rio_dma_transfer(struct file *filp, u32 transfer_mode,
 				xfer->offset, xfer->length);
 	}
 
-	nents = dma_map_sg(chan->device->dev,
-			   req->sgt.sgl, req->sgt.nents, dir);
-	if (nents == 0) {
+	ret = dma_map_sgtable(chan->device->dev, &req->sgt, dir, 0);
+	if (ret) {
 		rmcd_error("Failed to map SG list");
 		ret = -EFAULT;
 		goto err_pg;
 	}
 
-	ret = do_dma_request(req, xfer, sync, nents);
+	ret = do_dma_request(req, xfer, sync, req->sgt.nents);
 
 	if (ret >= 0) {
 		if (sync == RIO_TRANSFER_ASYNC)
-- 
2.17.1