linux-kernel - Re: Question: partial transfers of DMABUFs

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Date:   Wed, 15 Feb 2023 12:30:51 +0100
From:   Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
To:     Paul Cercueil <paul@...pouillou.net>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Vinod Koul <vkoul@...nel.org>
Cc:     dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org
Subject: Re: Question: partial transfers of DMABUFs

Hey,

On 2023-02-15 11:48, Paul Cercueil wrote:
> Hi,
>
> I am working on adding support for DMABUFs in the IIO subsystem.
>
> One thing we want there, is the ability to specify the number of bytes
> to transfer (while still defaulting to the DMABUF size).
>
> Since dma_buf_map_attachment() returns a sg_table, I basically have two
> options, and I can't decide which one is the best (or the less ugly):
>
> - Either I add a new API function similar to dmaengine_prep_slave_sg(),
> which still takes a scatterlist as argument but also takes the number
> of bytes as argument;
>
> - Or I add a function to duplicate the scatterlist and then shrink it
> manually, which doesn't sound like a good idea either.
>
> What would be the recommended way?

Does this need an api change? If you create a DMA-BUF of size X, it has 
to be of size X. You can pad with a dummy page probably if you know it 
in advance. But after it has been imported, it cannot change size.

You don´t have to write the entire dma-buf either, so if you want to 
create a 1GB buf and only use the first 4K, that is allowed. The 
contents of  the remainder of the DMA-BUF are undefined. It's up to 
userspace to assign a meaning to it.

I think I'm missing something here that makes the whole question m,ake 
more sense.

~Maarten