Re: [PATCH v7 31/36] staging: tegra-vde: fix common struct sg_table related issues

[Dmitry Osipenko]

Ð Fri, 19 Jun 2020 12:36:31 +0200
Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> ÐÐÑÐÑ:

> 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>
> Reviewed-by: Dmitry Osipenko <digetx@xxxxxxxxx>
> ---
>  drivers/staging/media/tegra-vde/iommu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/staging/media/tegra-vde/iommu.c
> b/drivers/staging/media/tegra-vde/iommu.c index
> 6af863d92123..adf8dc7ee25c 100644 ---
> a/drivers/staging/media/tegra-vde/iommu.c +++
> b/drivers/staging/media/tegra-vde/iommu.c @@ -36,8 +36,8 @@ int
> tegra_vde_iommu_map(struct tegra_vde *vde, 
>  	addr = iova_dma_addr(&vde->iova, iova);
>  
> -	size = iommu_map_sg(vde->domain, addr, sgt->sgl, sgt->nents,
> -			    IOMMU_READ | IOMMU_WRITE);
> +	size = iommu_map_sgtable(vde->domain, addr, sgt,
> +				 IOMMU_READ | IOMMU_WRITE);
>  	if (!size) {
>  		__free_iova(&vde->iova, iova);
>  		return -ENXIO;

Ahh, I saw the build failure report. You're changing the DMA API in
this series, while DMA API isn't used by this driver, it uses IOMMU
API. Hence there is no need to touch this code. Similar problem in the
host1x driver patch.