linux-kernel - Question: partial transfers of DMABUFs

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Date:   Wed, 15 Feb 2023 10:48:56 +0000
From:   Paul Cercueil <paul@...pouillou.net>
To:     Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Vinod Koul <vkoul@...nel.org>
Cc:     linux-media@...r.kernel.org, dmaengine@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Question: partial transfers of DMABUFs

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?

Cheers,
-Paul