linux-kernel - Re: [PATCH 2/2] dmabuf/heaps: implement DMA_BUF_IOCTL_RW_FILE for system

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Message-ID: <d786ff9f-9bf0-42e1-987f-f2091fd90279@amd.com>
Date: Wed, 21 May 2025 13:56:46 +0200
From: Christian König <christian.koenig@....com>
To: wangtao <tao.wangtao@...or.com>, "T.J. Mercier" <tjmercier@...gle.com>
Cc: "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
 "benjamin.gaignard@...labora.com" <benjamin.gaignard@...labora.com>,
 "Brian.Starkey@....com" <Brian.Starkey@....com>,
 "jstultz@...gle.com" <jstultz@...gle.com>,
 "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "wangbintian(BintianWang)" <bintian.wang@...or.com>,
 yipengxiang <yipengxiang@...or.com>, liulu 00013167 <liulu.liu@...or.com>,
 hanfeng 00012985 <feng.han@...or.com>,
 "amir73il@...il.com" <amir73il@...il.com>
Subject: Re: [PATCH 2/2] dmabuf/heaps: implement DMA_BUF_IOCTL_RW_FILE for
 system_heap

On 5/21/25 12:25, wangtao wrote:
> [wangtao] I previously explained that read/sendfile/splice/copy_file_range
> syscalls can't achieve dmabuf direct IO zero-copy.

And why can't you work on improving those syscalls instead of creating a new IOCTL?

> My focus is enabling dmabuf direct I/O for [regular file] <--DMA--> [dmabuf]
> zero-copy.

Yeah and that focus is wrong. You need to work on a general solution to the issue and not specific to your problem.

> Any API achieving this would work. Are there other uAPIs you think
> could help? Could you recommend experts who might offer suggestions?

Well once more: Either work on sendfile or copy_file_range or eventually splice to make it what you want to do.

When that is done we can discuss with the VFS people if that approach is feasible.

But just bypassing the VFS review by implementing a DMA-buf specific IOCTL is a NO-GO. That is clearly not something you can do in any way.

Regards,
Christian.