Re: [PATCH v9 12/32] drm: msm: fix common struct sg_table related issues

[Robin Murphy <robin.murphy@arm.com>]

On 2020-08-26 07:32, Marek Szyprowski wrote:
> 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>
> Acked-by: Rob Clark <robdclark@gmail.com>
> ---
>   drivers/gpu/drm/msm/msm_gem.c    | 13 +++++--------
>   drivers/gpu/drm/msm/msm_gpummu.c | 14 ++++++--------
>   drivers/gpu/drm/msm/msm_iommu.c  |  2 +-
>   3 files changed, 12 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> index b2f49152b4d4..8c7ae812b813 100644
> --- a/drivers/gpu/drm/msm/msm_gem.c
> +++ b/drivers/gpu/drm/msm/msm_gem.c
> @@ -53,11 +53,10 @@ static void sync_for_device(struct msm_gem_object *msm_obj)
>   	struct device *dev = msm_obj->base.dev->dev;
>   
>   	if (get_dma_ops(dev) && IS_ENABLED(CONFIG_ARM64)) {
> -		dma_sync_sg_for_device(dev, msm_obj->sgt->sgl,
> -			msm_obj->sgt->nents, DMA_BIDIRECTIONAL);
> +		dma_sync_sgtable_for_device(dev, msm_obj->sgt,
> +					    DMA_BIDIRECTIONAL);
>   	} else {
> -		dma_map_sg(dev, msm_obj->sgt->sgl,
> -			msm_obj->sgt->nents, DMA_BIDIRECTIONAL);
> +		dma_map_sgtable(dev, msm_obj->sgt, DMA_BIDIRECTIONAL, 0);
>   	}
>   }
>   
> @@ -66,11 +65,9 @@ static void sync_for_cpu(struct msm_gem_object *msm_obj)
>   	struct device *dev = msm_obj->base.dev->dev;
>   
>   	if (get_dma_ops(dev) && IS_ENABLED(CONFIG_ARM64)) {
> -		dma_sync_sg_for_cpu(dev, msm_obj->sgt->sgl,
> -			msm_obj->sgt->nents, DMA_BIDIRECTIONAL);
> +		dma_sync_sgtable_for_cpu(dev, msm_obj->sgt, DMA_BIDIRECTIONAL);
>   	} else {
> -		dma_unmap_sg(dev, msm_obj->sgt->sgl,
> -			msm_obj->sgt->nents, DMA_BIDIRECTIONAL);
> +		dma_unmap_sgtable(dev, msm_obj->sgt, DMA_BIDIRECTIONAL, 0);
>   	}
>   }
>   
> diff --git a/drivers/gpu/drm/msm/msm_gpummu.c b/drivers/gpu/drm/msm/msm_gpummu.c
> index 310a31b05faa..319f06c28235 100644
> --- a/drivers/gpu/drm/msm/msm_gpummu.c
> +++ b/drivers/gpu/drm/msm/msm_gpummu.c
> @@ -30,21 +30,19 @@ static int msm_gpummu_map(struct msm_mmu *mmu, uint64_t iova,
>   {
>   	struct msm_gpummu *gpummu = to_msm_gpummu(mmu);
>   	unsigned idx = (iova - GPUMMU_VA_START) / GPUMMU_PAGE_SIZE;
> -	struct scatterlist *sg;
> +	struct sg_dma_page_iter dma_iter;
>   	unsigned prot_bits = 0;
> -	unsigned i, j;
>   
>   	if (prot & IOMMU_WRITE)
>   		prot_bits |= 1;
>   	if (prot & IOMMU_READ)
>   		prot_bits |= 2;
>   
> -	for_each_sg(sgt->sgl, sg, sgt->nents, i) {
> -		dma_addr_t addr = sg->dma_address;
> -		for (j = 0; j < sg->length / GPUMMU_PAGE_SIZE; j++, idx++) {
> -			gpummu->table[idx] = addr | prot_bits;
> -			addr += GPUMMU_PAGE_SIZE;
> -		}
> +	for_each_sgtable_dma_page(sgt, &dma_iter, 0) {
> +		dma_addr_t addr = sg_page_iter_dma_address(&dma_iter);
> +
> +		BUILD_BUG_ON(GPUMMU_PAGE_SIZE != PAGE_SIZE);
> +		gpummu->table[idx++] = addr | prot_bits;

Given that the BUILD_BUG_ON might prevent valid arm64 configs from 
building, how about a simple tweak like:

		for (i = 0; i < PAGE_SIZE; i += GPUMMU_PAGE_SIZE)
			gpummu->table[idx++] = i + addr | prot_bits;
?

Or alternatively perhaps some more aggressive #ifdefs or makefile tweaks 
to prevent the GPUMMU code building for arm64 at all if it's only 
relevant to 32-bit platforms (which I believe might be the case).

Robin.

>   	}
>   
>   	/* we can improve by deferring flush for multiple map() */
> diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c
> index 3a381a9674c9..6c31e65834c6 100644
> --- a/drivers/gpu/drm/msm/msm_iommu.c
> +++ b/drivers/gpu/drm/msm/msm_iommu.c
> @@ -36,7 +36,7 @@ static int msm_iommu_map(struct msm_mmu *mmu, uint64_t iova,
>   	struct msm_iommu *iommu = to_msm_iommu(mmu);
>   	size_t ret;
>   
> -	ret = iommu_map_sg(iommu->domain, iova, sgt->sgl, sgt->nents, prot);
> +	ret = iommu_map_sgtable(iommu->domain, iova, sgt, prot);
>   	WARN_ON(!ret);
>   
>   	return (ret == len) ? 0 : -EINVAL;
> 
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