linux-kernel - Re: [RFC v2 01/11] file: add callback for pre-mapping dmabuf

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Message-ID: <7b2017f4-02a3-482a-a173-bb16b895c0cb@amd.com>
Date: Thu, 4 Dec 2025 11:46:45 +0100
From: Christian König <christian.koenig@....com>
To: Pavel Begunkov <asml.silence@...il.com>, linux-block@...r.kernel.org,
 io-uring@...r.kernel.org
Cc: Vishal Verma <vishal1.verma@...el.com>, tushar.gohad@...el.com,
 Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
 Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Sumit Semwal <sumit.semwal@...aro.org>, linux-kernel@...r.kernel.org,
 linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [RFC v2 01/11] file: add callback for pre-mapping dmabuf

On 11/23/25 23:51, Pavel Begunkov wrote:
> Add a file callback that maps a dmabuf for the given file and returns
> an opaque token of type struct dma_token representing the mapping.

I'm really scratching my head what you mean with that?

And why the heck would we need to pass a DMA-buf to a struct file?

Regards,
Christian.


> The
> implementation details are hidden from the caller, and the implementors
> are normally expected to extend the structure.
> 
> The callback callers will be able to pass the token with an IO request,
> which implemented in following patches as a new iterator type. The user
> should release the token once it's not needed by calling the provided
> release callback via appropriate helpers.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  include/linux/dma_token.h | 35 +++++++++++++++++++++++++++++++++++
>  include/linux/fs.h        |  4 ++++
>  2 files changed, 39 insertions(+)
>  create mode 100644 include/linux/dma_token.h
> 
> diff --git a/include/linux/dma_token.h b/include/linux/dma_token.h
> new file mode 100644
> index 000000000000..9194b34282c2
> --- /dev/null
> +++ b/include/linux/dma_token.h
> @@ -0,0 +1,35 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_DMA_TOKEN_H
> +#define _LINUX_DMA_TOKEN_H
> +
> +#include <linux/dma-buf.h>
> +
> +struct dma_token_params {
> +	struct dma_buf			*dmabuf;
> +	enum dma_data_direction		dir;
> +};
> +
> +struct dma_token {
> +	void (*release)(struct dma_token *);
> +};
> +
> +static inline void dma_token_release(struct dma_token *token)
> +{
> +	token->release(token);
> +}
> +
> +static inline struct dma_token *
> +dma_token_create(struct file *file, struct dma_token_params *params)
> +{
> +	struct dma_token *res;
> +
> +	if (!file->f_op->dma_map)
> +		return ERR_PTR(-EOPNOTSUPP);
> +	res = file->f_op->dma_map(file, params);
> +
> +	WARN_ON_ONCE(!IS_ERR(res) && !res->release);
> +
> +	return res;
> +}
> +
> +#endif
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index c895146c1444..0ce9a53fabec 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2262,6 +2262,8 @@ struct dir_context {
>  struct iov_iter;
>  struct io_uring_cmd;
>  struct offset_ctx;
> +struct dma_token;
> +struct dma_token_params;
>  
>  typedef unsigned int __bitwise fop_flags_t;
>  
> @@ -2309,6 +2311,8 @@ struct file_operations {
>  	int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *,
>  				unsigned int poll_flags);
>  	int (*mmap_prepare)(struct vm_area_desc *);
> +	struct dma_token *(*dma_map)(struct file *,
> +				     struct dma_token_params *);
>  } __randomize_layout;
>  
>  /* Supports async buffered reads */