Message-ID: <d540965a25138772fa063d62e907ffd611f93205.camel@crapouillou.net>
Subject: Re: Question: partial transfers of DMABUFs
From:   Paul Cercueil <paul@crapouillou.net>
To:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Christian =?ISO-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        Vinod Koul <vkoul@kernel.org>
Cc:     dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org
Date:   Wed, 15 Feb 2023 11:47:55 +0000
In-Reply-To: <836d600a-bb1c-fbb2-89f5-7c79c3150e8c@linux.intel.com>
References: <53ea4d2db570d3ca514a69015488bd5b849a5193.camel@crapouillou.net>
         <836d600a-bb1c-fbb2-89f5-7c79c3150e8c@linux.intel.com>
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Hi Maarten,

Le mercredi 15 f=C3=A9vrier 2023 =C3=A0 12:30 +0100, Maarten Lankhorst a =
=C3=A9crit=C2=A0:
> Hey,
>=20
> On 2023-02-15 11:48, Paul Cercueil wrote:
> > Hi,
> >=20
> > I am working on adding support for DMABUFs in the IIO subsystem.
> >=20
> > One thing we want there, is the ability to specify the number of
> > bytes
> > to transfer (while still defaulting to the DMABUF size).
> >=20
> > 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):
> >=20
> > - 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;
> >=20
> > - Or I add a function to duplicate the scatterlist and then shrink
> > it
> > manually, which doesn't sound like a good idea either.
> >=20
> > What would be the recommended way?
>=20
> Does this need an api change? If you create a DMA-BUF of size X, it
> has=20
> to be of size X. You can pad with a dummy page probably if you know
> it=20
> in advance. But after it has been imported, it cannot change size.

Yes, the sizes are fixed.

> You don=C2=B4t have to write the entire dma-buf either, so if you want to=
=20
> create a 1GB buf and only use the first 4K, that is allowed. The=20
> contents of=C2=A0 the remainder of the DMA-BUF are undefined. It's up to=
=20
> userspace to assign a meaning to it.
>=20
> I think I'm missing something here that makes the whole question
> m,ake=20
> more sense.

I want my userspace to be able to specify how much of the DMABUF is to
be read from or written to.

So in my new "dmabuf enqueue" IOCTL that I want to add to IIO, I added
a parameter to specify the number of bytes to transfer (where 0 means
the whole buffer).

The problem I have now, is that the current dmaengine core does not
have a API function that takes a scatterlist (returned by
dma_map_attachment()) and a transfer size in bytes, it will always
transfer the whole scatterlist.

So my two options would be to add a new API function to support
specifying a bytes count, or add a mechanism to duplicate a
scatterlist, so that I can tweak it to the right size.

> ~Maarten

Cheers,
-Paul