linux-kernel - Re: [PATCH v9 28/32] misc: fastrpc: fix common struct sg

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Date:   Tue, 1 Sep 2020 21:14:06 +0100
From:   Robin Murphy <robin.murphy@....com>
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:     Christoph Hellwig <hch@....de>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-arm-kernel@...ts.infradead.org,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v9 28/32] misc: fastrpc: fix common struct sg_table
 related issues

On 2020-08-26 07:33, 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.

Reviewed-by: Robin Murphy <robin.murphy@....com>

> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
>   drivers/misc/fastrpc.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 7939c55daceb..9d6867749316 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -518,7 +518,7 @@ fastrpc_map_dma_buf(struct dma_buf_attachment *attachment,
>   
>   	table = &a->sgt;
>   
> -	if (!dma_map_sg(attachment->dev, table->sgl, table->nents, dir))
> +	if (!dma_map_sgtable(attachment->dev, table, dir, 0))
>   		return ERR_PTR(-ENOMEM);
>   
>   	return table;
> @@ -528,7 +528,7 @@ static void fastrpc_unmap_dma_buf(struct dma_buf_attachment *attach,
>   				  struct sg_table *table,
>   				  enum dma_data_direction dir)
>   {
> -	dma_unmap_sg(attach->dev, table->sgl, table->nents, dir);
> +	dma_unmap_sgtable(attach->dev, table, dir, 0);
>   }
>   
>   static void fastrpc_release(struct dma_buf *dmabuf)
>