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

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Message-ID: <0571ca61-7b17-4167-83eb-4269bd0459fe@amd.com>
Date: Thu, 4 Dec 2025 12:09:46 +0100
From: Christian König <christian.koenig@....com>
To: Christoph Hellwig <hch@....de>
Cc: Pavel Begunkov <asml.silence@...il.com>, linux-block@...r.kernel.org,
 io-uring@...r.kernel.org, Vishal Verma <vishal1.verma@...el.com>,
 tushar.gohad@...el.com, Keith Busch <kbusch@...nel.org>,
 Jens Axboe <axboe@...nel.dk>, 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 12/4/25 12:07, Christoph Hellwig wrote:
> On Thu, Dec 04, 2025 at 11:46:45AM +0100, Christian König wrote:
>> 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?
> 
> I find the naming pretty confusing a well.  But what this does is to
> tell the file system/driver that it should expect a future
> read_iter/write_iter operation that takes data from / puts data into
> the dmabuf passed to this operation.

That explanation makes much more sense.

The remaining question is why does the underlying file system / driver needs to know that it will get addresses from a DMA-buf?

Regards,
Christian.