linux-kernel - Re: [Linaro-mm-sig] Re: [PATCH v5 1/6] dma-buf: Add dma_buf_{begin,end}

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Date: Tue, 30 Jan 2024 11:40:38 +0100
From: Daniel Vetter <daniel@...ll.ch>
To: Paul Cercueil <paul@...pouillou.net>
Cc: Christian König <christian.koenig@....com>,
	Christian König <ckoenig.leichtzumerken@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jonathan Corbet <corbet@....net>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Jonathan Cameron <jic23@...nel.org>,
	Nuno Sá <noname.nuno@...il.com>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	linux-usb@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	Christoph Hellwig <hch@....de>
Subject: Re: [Linaro-mm-sig] Re: [PATCH v5 1/6] dma-buf: Add
 dma_buf_{begin,end}_access()

On Tue, Jan 30, 2024 at 10:48:23AM +0100, Paul Cercueil wrote:
> Le mardi 30 janvier 2024 à 10:23 +0100, Christian König a écrit :
> >  I would say we start with the DMA-API by getting away from sg_tables
> > to something cleaner and state oriented. 
> 
> FYI I am already adding a 'dma_vec' object in my IIO DMABUF patchset,
> which is just a dead simple
> 
> struct dma_vec {
>   dma_addr_t addr;
>   size_t len;
> };
> 
> (The rationale for introducing it in the IIO DMABUF patchset was that
> the "scatterlist" wouldn't allow me to change the transfer size.)
> 
> So I believe a new "sg_table"-like could just be an array of struct
> dma_vec + flags.

Yeah that's pretty much the proposal I've seen, split the sg table into
input data (struct page + len) and output data (which is the dma_addr_t +
len you have above).

The part I don't expect to ever happen, because it hasn't the past 20 or
so years, is that the dma-api will give us information about what is
needed to keep the buffers coherency between various devices and the cpu.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch