linux-kernel - [PATCH v5 26/38] drm: vmwgfx: fix common struct sg

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Message-Id: <20200513133245.6408-26-m.szyprowski@samsung.com>
Date:   Wed, 13 May 2020 15:32:33 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     dri-devel@...ts.freedesktop.org, iommu@...ts.linux-foundation.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-arm-kernel@...ts.infradead.org,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>
Subject: [PATCH v5 26/38] drm: vmwgfx: fix common struct sg_table related
 issues

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@...sung.com>
Acked-by: Roland Scheidegger <sroland@...are.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/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
index bf0bc46..49ed6ad 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
@@ -362,8 +362,7 @@ static void vmw_ttm_unmap_from_dma(struct vmw_ttm_tt *vmw_tt)
 {
 	struct device *dev = vmw_tt->dev_priv->dev->dev;
 
-	dma_unmap_sg(dev, vmw_tt->sgt.sgl, vmw_tt->sgt.nents,
-		DMA_BIDIRECTIONAL);
+	dma_unmap_sgtable(dev, vmw_tt->sgt, DMA_BIDIRECTIONAL, 0);
 	vmw_tt->sgt.nents = vmw_tt->sgt.orig_nents;
 }
 
@@ -383,16 +382,8 @@ static void vmw_ttm_unmap_from_dma(struct vmw_ttm_tt *vmw_tt)
 static int vmw_ttm_map_for_dma(struct vmw_ttm_tt *vmw_tt)
 {
 	struct device *dev = vmw_tt->dev_priv->dev->dev;
-	int ret;
-
-	ret = dma_map_sg(dev, vmw_tt->sgt.sgl, vmw_tt->sgt.orig_nents,
-			 DMA_BIDIRECTIONAL);
-	if (unlikely(ret == 0))
-		return -ENOMEM;
 
-	vmw_tt->sgt.nents = ret;
-
-	return 0;
+	return dma_map_sgtable(dev, vmw_tt->sgt, DMA_BIDIRECTIONAL, 0);
 }
 
 /**
@@ -449,10 +440,10 @@ static int vmw_ttm_map_dma(struct vmw_ttm_tt *vmw_tt)
 		if (unlikely(ret != 0))
 			goto out_sg_alloc_fail;
 
-		if (vsgt->num_pages > vmw_tt->sgt.nents) {
+		if (vsgt->num_pages > vmw_tt->sgt.orig_nents) {
 			uint64_t over_alloc =
 				sgl_size * (vsgt->num_pages -
-					    vmw_tt->sgt.nents);
+					    vmw_tt->sgt.orig_nents);
 
 			ttm_mem_global_free(glob, over_alloc);
 			vmw_tt->sg_alloc_size -= over_alloc;
-- 
1.9.1