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()

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 <>
v8: fixed issues pointed by kbuilt test robot (use of uninitialized
 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 
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)
        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);
        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)

dri-devel mailing list

Reply via email to