Commit 34a4e66f authored Apr 28, 2020 by Marek Szyprowski

drm: panfrost: 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@samsung.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>

parent 53760655

drivers/gpu/drm/panfrost/panfrost_gem.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -41,8 +41,8 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)

		for (i = 0; i < n_sgt; i++) {
		if (bo->sgts[i].sgl) {
		dma_unmap_sg(pfdev->dev, bo->sgts[i].sgl,
		bo->sgts[i].nents, DMA_BIDIRECTIONAL);
		dma_unmap_sgtable(pfdev->dev, &bo->sgts[i],
		DMA_BIDIRECTIONAL, 0);
		sg_free_table(&bo->sgts[i]);
		}
		}

drivers/gpu/drm/panfrost/panfrost_mmu.c

+3 −4

Original line number	Diff line number	Diff line
		@@ -253,7 +253,7 @@ static int mmu_map_sg(struct panfrost_device pfdev, struct panfrost_mmu mmu,
		struct io_pgtable_ops *ops = mmu->pgtbl_ops;
		u64 start_iova = iova;

		for_each_sg(sgt->sgl, sgl, sgt->nents, count) {
		for_each_sgtable_dma_sg(sgt, sgl, count) {
		unsigned long paddr = sg_dma_address(sgl);
		size_t len = sg_dma_len(sgl);

		@@ -517,10 +517,9 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
		if (ret)
		goto err_pages;

		if (!dma_map_sg(pfdev->dev, sgt->sgl, sgt->nents, DMA_BIDIRECTIONAL)) {
		ret = -EINVAL;
		ret = dma_map_sgtable(pfdev->dev, sgt, DMA_BIDIRECTIONAL, 0);
		if (ret)
		goto err_map;
		}

		mmu_map_sg(pfdev, bomapping->mmu, addr,
		IOMMU_WRITE \| IOMMU_READ \| IOMMU_NOEXEC, sgt);

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