Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Marek Szyprowski <m.szyprowski@samsung.com>
To:     dri-devel@lists.freedesktop.org, iommu@lists.linux-foundation.org,
        linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org
Cc:     Marek Szyprowski <m.szyprowski@samsung.com>,
        Christoph Hellwig <hch@lst.de>,
        Robin Murphy <robin.murphy@arm.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        linux-arm-kernel@lists.infradead.org,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Matt Porter <mporter@kernel.crashing.org>,
        Alexandre Bounine <alex.bou9@gmail.com>
Subject: [PATCH v5 35/38] rapidio: fix common struct sg_table related issues
Date:   Wed, 13 May 2020 15:32:42 +0200
Message-Id: <20200513133245.6408-35-m.szyprowski@samsung.com>
In-Reply-To: <20200513133245.6408-1-m.szyprowski@samsung.com>
Content-Type: text/plain; charset="utf-8"
CMS-TYPE: 201P
References: <20200513132114.6046-1-m.szyprowski@samsung.com>
        <20200513133245.6408-1-m.szyprowski@samsung.com>
        <CGME20200513133323eucas1p1344fe42c8710f0ed84cf3c9fdb5ca515@eucas1p1.samsung.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@samsung.com>
---
For more information, see '[PATCH v5 00/38] DRM: fix struct sg_table nents
vs. orig_nents misuse' thread:
https://lore.kernel.org/linux-iommu/20200513132114.6046-1-m.szyprowski@samsung.com/T/
---
 drivers/rapidio/devices/rio_mport_cdev.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/rapidio/devices/rio_mport_cdev.c b/drivers/rapidio/devices/rio_mport_cdev.c
index 4029637..92d8742 100644
--- a/drivers/rapidio/devices/rio_mport_cdev.c
+++ b/drivers/rapidio/devices/rio_mport_cdev.c
@@ -574,8 +574,7 @@ static void dma_req_free(struct kref *ref)
 	struct mport_cdev_priv *priv = req->priv;
 	unsigned int i;
 
-	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) {
 		for (i = 0; i < req->nr_pages; i++)
@@ -927,9 +926,8 @@ static int do_dma_request(struct mport_dma_req *req,
 				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;
-- 
1.9.1