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Date: Wed, 7 Jun 2023 19:36:43 +0200
From: Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To: Christoph Hellwig <hch@infradead.org>
Cc: iommu@lists.linux.dev, Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: Scatter-gather DMA
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On Wed, 7 Jun 2023 06:51:56 -0700
Christoph Hellwig <hch@infradead.org> wrote:

> On Tue, Jun 06, 2023 at 04:22:01PM +0200, Petr Tesa=C5=99=C3=ADk wrote:
> > Is there any reason to use struct page except that the SG list API is
> > based on page lists? =20
>=20
> That's how the scatterlist was historically done, and we have plenty
> of places that use the scatterlist for accessing the data using the
> page pointer.
>=20
> Looking back the scatterlist was a rather bad idea as it mixes the
> kernel in-memory representation (page, offset, len) with the DMA
> information dma_address, dma_len.   I've been proposing forever to
> split these two and just use a bio_vec for the kernel mapping
> representation and a new phys_vec for the dma_address, dma_len tuples.
>=20
> And while I've only been talking, Jason apparently already has working
> prototype code for this, but I've not seen it yet.

Thanks for the explanation. The important new piece of information is
that somebody (Jason) is already taking care, so I will not interfere.

Petr T