linux-kernel - Re: [PATCH v3 01/25] dma-mapping: add generic helpers for mapping sgtable objects

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Date:   Thu, 7 May 2020 10:47:42 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Marek Szyprowski <m.szyprowski@...sung.com>,
        dri-devel@...ts.freedesktop.org, iommu@...ts.linux-foundation.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Cc:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        David Airlie <airlied@...ux.ie>,
        Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 01/25] dma-mapping: add generic helpers for mapping
 sgtable objects

Hi Marek,

On 05/05/2020 10:45, Marek Szyprowski wrote:
> struct sg_table is a common structure used for describing a memory
> buffer. 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 pages (nents entry).
> 
> It turned out that it was a common mistake to misuse nents and orig_nents
> entries, call 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 introduce a common wrappers operating directly
> on the struct sg_table objects, which take care of the proper use of
> the nents and orig_nents entries.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
>  include/linux/dma-mapping.h | 32 ++++++++++++++++++++++++++++++++
>  include/linux/iommu.h       |  6 ++++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index b43116a..8364c20d 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -609,6 +609,36 @@ static inline void dma_sync_single_range_for_device(struct device *dev,
>  	return dma_sync_single_for_device(dev, addr + offset, size, dir);
>  }
>  
> +static inline int dma_map_sgtable_attrs(struct device *dev,
> +	struct sg_table *sgt, enum dma_data_direction dir, unsigned long attrs)

Please document these new inline functions!

The core reason IMHO why so many drivers do this wrong is that the struct sg_table
documentation is very poor. In particular w.r.t. how the nents and orig_nents
fields should be used.

It confused me, and clearly I am not alone in that.

So while this is a very nice improvement, it doesn't address the core issue, i.e.
the documentation.

Especially some examples on how to use these new functions would be welcome.

Regards,

	Hans

> +{
> +	int n = dma_map_sg_attrs(dev, sgt->sgl, sgt->orig_nents, dir, attrs);
> +
> +	if (n > 0) {
> +		sgt->nents = n;
> +		return 0;
> +	}
> +	return -EINVAL;
> +}
> +
> +static inline void dma_unmap_sgtable_attrs(struct device *dev,
> +	struct sg_table *sgt, enum dma_data_direction dir, unsigned long attrs)
> +{
> +	dma_unmap_sg_attrs(dev, sgt->sgl, sgt->orig_nents, dir, attrs);
> +}
> +
> +static inline void dma_sync_sgtable_for_cpu(struct device *dev,
> +	struct sg_table *sgt, enum dma_data_direction dir)
> +{
> +	dma_sync_sg_for_cpu(dev, sgt->sgl, sgt->orig_nents, dir);
> +}
> +
> +static inline void dma_sync_sgtable_for_device(struct device *dev,
> +	struct sg_table *sgt, enum dma_data_direction dir)
> +{
> +	dma_sync_sg_for_device(dev, sgt->sgl, sgt->orig_nents, dir);
> +}
> +
>  #define dma_map_single(d, a, s, r) dma_map_single_attrs(d, a, s, r, 0)
>  #define dma_unmap_single(d, a, s, r) dma_unmap_single_attrs(d, a, s, r, 0)
>  #define dma_map_sg(d, s, n, r) dma_map_sg_attrs(d, s, n, r, 0)
> @@ -617,6 +647,8 @@ static inline void dma_sync_single_range_for_device(struct device *dev,
>  #define dma_unmap_page(d, a, s, r) dma_unmap_page_attrs(d, a, s, r, 0)
>  #define dma_get_sgtable(d, t, v, h, s) dma_get_sgtable_attrs(d, t, v, h, s, 0)
>  #define dma_mmap_coherent(d, v, c, h, s) dma_mmap_attrs(d, v, c, h, s, 0)
> +#define dma_map_sgtable(d, s, r) dma_map_sgtable_attrs(d, s, r, 0)
> +#define dma_unmap_sgtable(d, s, r) dma_unmap_sgtable_attrs(d, s, r, 0)
>  
>  extern int dma_common_mmap(struct device *dev, struct vm_area_struct *vma,
>  		void *cpu_addr, dma_addr_t dma_addr, size_t size,
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 7ef8b0b..5106b65 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -466,6 +466,12 @@ extern size_t iommu_map_sg_atomic(struct iommu_domain *domain,
>  extern void iommu_set_fault_handler(struct iommu_domain *domain,
>  			iommu_fault_handler_t handler, void *token);
>  
> +static inline size_t iommu_map_sgtable(struct iommu_domain *domain,
> +			unsigned long iova, struct sg_table *sgt, int prot)
> +{
> +	return iommu_map_sg(domain, iova, sgt->sgl, sgt->orig_nents, prot);
> +}
> +
>  extern void iommu_get_resv_regions(struct device *dev, struct list_head *list);
>  extern void iommu_put_resv_regions(struct device *dev, struct list_head *list);
>  extern void generic_iommu_put_resv_regions(struct device *dev,
>