Message-ID: <c5fe8089-49e9-2bec-eac5-e8a035412cf3@amd.com>
Date:   Wed, 15 Feb 2023 13:58:08 +0100
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Subject: Re: Question: partial transfers of DMABUFs
Content-Language: en-US
To:     Paul Cercueil <paul@crapouillou.net>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Vinod Koul <vkoul@kernel.org>
Cc:     linux-media@vger.kernel.org, dmaengine@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
References: <53ea4d2db570d3ca514a69015488bd5b849a5193.camel@crapouillou.net>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
In-Reply-To: <53ea4d2db570d3ca514a69015488bd5b849a5193.camel@crapouillou.net>
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Hi Paul,

Am 15.02.23 um 11:48 schrieb Paul Cercueil:
> 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,

Please don't assume that this is an sg_table. We just used it as 
container for DMA addresses, but this has proven to be a mistake.

There is work underway to replace the sg_table with (for example) just 
an array of DMA addresses.

> 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?

I strongly recommend to come up with a new function which only takes DMA 
addresses and separate segment length.

Regards,
Christian.

>
> Cheers,
> -Paul