linux-kernel - Re: [PATCH 4/5] udmabuf: add get_sg

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Message-ID: <98122315-37d4-457a-9c71-14e4d2c70062@amd.com>
Date: Thu, 1 Aug 2024 12:56:30 +0200
From: Christian König <christian.koenig@....com>
To: Huan Yang <link@...o.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Sumit Semwal <sumit.semwal@...aro.org>, dri-devel@...ts.freedesktop.org,
 linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
 linux-kernel@...r.kernel.org
Cc: opensource.kernel@...o.com
Subject: Re: [PATCH 4/5] udmabuf: add get_sg_table helper function

Am 01.08.24 um 12:45 schrieb Huan Yang:
> Currently, there are three duplicate pieces of code that retrieve
> sg_table and update uduf->sg.
>
> Since the sgt is used to populate the page in both mmap and vmap.It is
> necessary to ensure that ubuf->sg is set correctly.

That is a really bad idea. Why are sg tables used to populated the page 
tables?

Regards,
Christian.


>
> This patch add a helper function, if ubuf->sg exist, just return it.
> Or else, try alloc a new sgt, and cmpxchg to set it.
>
> When the swap fails, it means that another process has set sg correctly.
> Therefore, we reuse the new sg. If trigger by device, need invoke map to
> sync it.
>
> Signed-off-by: Huan Yang <link@...o.com>
> ---
>   drivers/dma-buf/udmabuf.c | 60 ++++++++++++++++++++++++++++-----------
>   1 file changed, 43 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index 7ed532342d7f..677ebb2d462f 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -38,8 +38,9 @@ struct udmabuf_folio {
>   	struct list_head list;
>   };
>   
> -static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
> -				     enum dma_data_direction direction);
> +static struct sg_table *udmabuf_get_sg_table(struct device *dev,
> +					     struct dma_buf *buf,
> +					     enum dma_data_direction direction);
>   
>   static int mmap_udmabuf(struct dma_buf *buf, struct vm_area_struct *vma)
>   {
> @@ -52,12 +53,9 @@ static int mmap_udmabuf(struct dma_buf *buf, struct vm_area_struct *vma)
>   	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
>   		return -EINVAL;
>   
> -	if (!table) {
> -		table = get_sg_table(NULL, buf, 0);
> -		if (IS_ERR(table))
> -			return PTR_ERR(table);
> -		ubuf->sg = table;
> -	}
> +	table = udmabuf_get_sg_table(NULL, buf, 0);
> +	if (IS_ERR(table))
> +		return PTR_ERR(table);
>   
>   	for_each_sgtable_page(table, &piter, vma->vm_pgoff) {
>   		struct page *page = sg_page_iter_page(&piter);
> @@ -84,12 +82,9 @@ static int vmap_udmabuf(struct dma_buf *buf, struct iosys_map *map)
>   
>   	dma_resv_assert_held(buf->resv);
>   
> -	if (!sg) {
> -		sg = get_sg_table(NULL, buf, 0);
> -		if (IS_ERR(sg))
> -			return PTR_ERR(sg);
> -		ubuf->sg = sg;
> -	}
> +	sg = udmabuf_get_sg_table(NULL, buf, 0);
> +	if (IS_ERR(sg))
> +		return PTR_ERR(sg);
>   
>   	pages = kvmalloc_array(ubuf->pagecount, sizeof(*pages), GFP_KERNEL);
>   	if (!pages)
> @@ -154,6 +149,39 @@ static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
>   	return ERR_PTR(ret);
>   }
>   
> +static struct sg_table *udmabuf_get_sg_table(struct device *dev,
> +					     struct dma_buf *buf,
> +					     enum dma_data_direction direction)
> +{
> +	struct udmabuf *ubuf = buf->priv;
> +	struct sg_table *sg = READ_ONCE(ubuf->sg);
> +	int ret = 0;
> +
> +	if (sg)
> +		return sg;
> +
> +	sg = get_sg_table(dev, buf, direction);
> +	if (IS_ERR(sg))
> +		return sg;
> +
> +	// Success update ubuf's sg, just return.
> +	if (!cmpxchg(&ubuf->sg, NULL, sg))
> +		return sg;
> +
> +	// use the new sg table.
> +	sg_free_table(sg);
> +	kfree(sg);
> +	sg = READ_ONCE(ubuf->sg);
> +
> +	if (dev)
> +		ret = dma_map_sgtable(dev, sg, direction, 0);
> +
> +	if (ret < 0)
> +		return ERR_PTR(ret);
> +
> +	return sg;
> +}
> +
>   static void put_sg_table(struct device *dev, struct sg_table *sg,
>   			 enum dma_data_direction direction)
>   {
> @@ -230,12 +258,10 @@ static int begin_cpu_udmabuf(struct dma_buf *buf,
>   		return 0;
>   	}
>   
> -	sg = get_sg_table(dev, buf, direction);
> +	sg = udmabuf_get_sg_table(dev, buf, direction);
>   	if (IS_ERR(sg))
>   		return PTR_ERR(sg);
>   
> -	ubuf->sg = sg;
> -
>   	return 0;
>   }
>