linux-kernel - Re: [PATCH] dma-buf: add DMA_BUF_IOCTL_SYNC

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Message-ID: <CABdmKX1OZ9yT3YQA9e7JzKND9wfiL-hnf87Q6v9pwtnAeLHrdA@mail.gmail.com>
Date: Tue, 9 Apr 2024 09:37:07 -0700
From: "T.J. Mercier" <tjmercier@...gle.com>
To: Rong Qianfeng <11065417@...o.com>
Cc: Christian König <christian.koenig@....com>, 
	Rong Qianfeng <rongqianfeng@...o.com>, Jianqun Xu <jay.xu@...k-chips.com>, sumit.semwal@...aro.org, 
	pekka.paalanen@...labora.com, daniel.vetter@...ll.ch, jason@...kstrand.net, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org, 
	linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH] dma-buf: add DMA_BUF_IOCTL_SYNC_PARTIAL support

On Tue, Apr 9, 2024 at 12:34 AM Rong Qianfeng <11065417@...o.com> wrote:
>
>
> 在 2024/4/8 15:58, Christian König 写道:
> > Am 07.04.24 um 09:50 schrieb Rong Qianfeng:
> >> [SNIP]
> >>> Am 13.11.21 um 07:22 schrieb Jianqun Xu:
> >>>> Add DMA_BUF_IOCTL_SYNC_PARTIAL support for user to sync dma-buf with
> >>>> offset and len.
> >>>
> >>> You have not given an use case for this so it is a bit hard to
> >>> review. And from the existing use cases I don't see why this should
> >>> be necessary.
> >>>
> >>> Even worse from the existing backend implementation I don't even see
> >>> how drivers should be able to fulfill this semantics.
> >>>
> >>> Please explain further,
> >>> Christian.
> >> Here is a practical case:
> >> The user space can allocate a large chunk of dma-buf for
> >> self-management, used as a shared memory pool.
> >> Small dma-buf can be allocated from this shared memory pool and
> >> released back to it after use, thus improving the speed of dma-buf
> >> allocation and release.
> >> Additionally, custom functionalities such as memory statistics and
> >> boundary checking can be implemented in the user space.
> >> Of course, the above-mentioned functionalities require the
> >> implementation of a partial cache sync interface.
> >
> > Well that is obvious, but where is the code doing that?
> >
> > You can't send out code without an actual user of it. That will
> > obviously be rejected.
> >
> > Regards,
> > Christian.
>
> In fact, we have already used the user-level dma-buf memory pool in the
> camera shooting scenario on the phone.
>
>  From the test results, The execution time of the photo shooting
> algorithm has been reduced from 3.8s to 3s.
>
For phones, the (out of tree) Android version of the system heap has a
page pool connected to a shrinker. That allows you to skip page
allocation without fully pinning the memory like you get when
allocating a dma-buf that's way larger than necessary. If it's for a
camera maybe you need physically contiguous memory, but it's also
possible to set that up.

https://android.googlesource.com/kernel/common/+/refs/heads/android14-6.1/drivers/dma-buf/heaps/system_heap.c#377


> To be honest, I didn't understand your concern "...how drivers should be
> able to fulfill this semantics." Can you please help explain it in more
> detail?
>
> Thanks,
>
> Rong Qianfeng.
>
> >
> >>
> >> Thanks
> >> Rong Qianfeng.
> >
>