Re: [PATCH 2/3] dma-buf: Add DMA_BUF_IOCTL_GET_DMA_ADDR

Nicolas Dufresne
2025-04-11T18:34:39Z

Hi Bastien,

Le jeudi 10 avril 2025 à 16:53 +0200, Bastien Curutchet a écrit :
> There is no way to transmit the DMA address of a buffer to userspace.
> Some UIO users need this to handle DMA from userspace.

To me this API is against all safe practice we've been pushing forward
and has no place in DMA_BUF API.

If this is fine for the UIO subsystem to pass around physicial
addresses, then make this part of the UIO device ioctl.

regards,
Nicolas

>
> Add a new dma_buf_ops operation that returns the DMA address.
> Add a new ioctl to transmit this DMA address to userspace.
>
> Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
> ---
>  drivers/dma-buf/dma-buf.c    | 21 +++++++++++++++++++++
>  include/linux/dma-buf.h      |  1 +
>  include/uapi/linux/dma-buf.h |  1 +
>  3 files changed, 23 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index
> 398418bd9731ad7a3a1f12eaea6a155fa77a22fe..cbbb518981e54e50f479c3d1fcf
> 6da6971f639c1 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -454,6 +454,24 @@ static long dma_buf_import_sync_file(struct
> dma_buf *dmabuf,
>  }
>  #endif
>
> +static int dma_buf_get_dma_addr(struct dma_buf *dmabuf, u64 __user
> *arg)
> +{
> + u64 addr;
> + int ret;
> +
> + if (!dmabuf->ops->get_dma_addr)
> + return -EINVAL;
> +
> + ret = dmabuf->ops->get_dma_addr(dmabuf, &addr);
> + if (ret)
> + return ret;
> +
> + if (copy_to_user(arg, &addr, sizeof(u64)))
> + return -EFAULT;
> +
> + return 0;
> +}
> +
>  static long dma_buf_ioctl(struct file *file,
>     unsigned int cmd, unsigned long arg)
>  {
> @@ -504,6 +522,9 @@ static long dma_buf_ioctl(struct file *file,
>   return dma_buf_import_sync_file(dmabuf, (const void
> __user *)arg);
>  #endif
>
> + case DMA_BUF_IOCTL_GET_DMA_ADDR:
> + return dma_buf_get_dma_addr(dmabuf, (u64 __user
> *)arg);
> +
>   default:
>   return -ENOTTY;
>   }
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index
> 36216d28d8bdc01a9c9c47e27c392413f7f6c5fb..ed4bf15d3ce82e7a86323fff459
> 699a9bc8baa3b 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -285,6 +285,7 @@ struct dma_buf_ops {
>
>   int (*vmap)(struct dma_buf *dmabuf, struct iosys_map *map);
>   void (*vunmap)(struct dma_buf *dmabuf, struct iosys_map
> *map);
> + int (*get_dma_addr)(struct dma_buf *dmabuf, u64 *addr);
>  };
>
>  /**
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-
> buf.h
> index
> 5a6fda66d9adf01438619e7e67fa69f0fec2d88d..f3aba46942042de6a2e3a4cca3e
> b3f87175e29c9 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -178,5 +178,6 @@ struct dma_buf_import_sync_file {
>  #define DMA_BUF_SET_NAME_B _IOW(DMA_BUF_BASE, 1, __u64)
>  #define DMA_BUF_IOCTL_EXPORT_SYNC_FILE _IOWR(DMA_BUF_BASE, 2,
> struct dma_buf_export_sync_file)
>  #define DMA_BUF_IOCTL_IMPORT_SYNC_FILE _IOW(DMA_BUF_BASE, 3, struct
> dma_buf_import_sync_file)
> +#define DMA_BUF_IOCTL_GET_DMA_ADDR _IOR(DMA_BUF_BASE, 4, __u64
> *)
>
>  #endif