[PATCH v4 27/38] xen: gntdev: fix common struct sg_table related issues

[Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>]

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>
---
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/xen/gntdev-dmabuf.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/xen/gntdev-dmabuf.c b/drivers/xen/gntdev-dmabuf.c
index 75d3bb9..ba6cad8 100644
--- a/drivers/xen/gntdev-dmabuf.c
+++ b/drivers/xen/gntdev-dmabuf.c
@@ -247,10 +247,9 @@ static void dmabuf_exp_ops_detach(struct dma_buf *dma_buf,
 
                if (sgt) {
                        if (gntdev_dmabuf_attach->dir != DMA_NONE)
-                               dma_unmap_sg_attrs(attach->dev, sgt->sgl,
-                                                  sgt->nents,
-                                                  gntdev_dmabuf_attach->dir,
-                                                  DMA_ATTR_SKIP_CPU_SYNC);
+                               dma_unmap_sgtable(attach->dev, sgt,
+                                                 gntdev_dmabuf_attach->dir,
+                                                 DMA_ATTR_SKIP_CPU_SYNC);
                        sg_free_table(sgt);
                }
 
@@ -288,8 +287,8 @@ static void dmabuf_exp_ops_detach(struct dma_buf *dma_buf,
        sgt = dmabuf_pages_to_sgt(gntdev_dmabuf->pages,
                                  gntdev_dmabuf->nr_pages);
        if (!IS_ERR(sgt)) {
-               if (!dma_map_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,
-                                     DMA_ATTR_SKIP_CPU_SYNC)) {
+               if (dma_map_sgtable(attach->dev, sgt, dir,
+                                   DMA_ATTR_SKIP_CPU_SYNC)) {
                        sg_free_table(sgt);
                        kfree(sgt);
                        sgt = ERR_PTR(-ENOMEM);
@@ -625,7 +624,7 @@ static struct gntdev_dmabuf *dmabuf_imp_alloc_storage(int 
count)
 
        /* Now convert sgt to array of pages and check for page validity. */
        i = 0;
-       for_each_sg_page(sgt->sgl, &sg_iter, sgt->nents, 0) {
+       for_each_sgtable_page(sgt, &sg_iter, 0) {
                struct page *page = sg_page_iter_page(&sg_iter);
                /*
                 * Check if page is valid: this can happen if we are given
-- 
1.9.1