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Date: Wed, 10 Jul 2024 08:27:04 +0200
From: Christoph Hellwig <hch@lst.de>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Christoph Hellwig <hch@lst.de>, Leon Romanovsky <leon@kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Robin Murphy <robin.murphy@arm.com>,
	Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
	Keith Busch <kbusch@kernel.org>, "Zeng, Oak" <oak.zeng@intel.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Sagi Grimberg <sagi@grimberg.me>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Logan Gunthorpe <logang@deltatee.com>,
	Yishai Hadas <yishaih@nvidia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>,
	Kevin Tian <kevin.tian@intel.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	=?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
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	linux-rdma@vger.kernel.org, iommu@lists.linux.dev,
	linux-nvme@lists.infradead.org, linux-pci@vger.kernel.org,
	kvm@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API
Message-ID: <20240710062704.GA25953@lst.de>
References: <cover.1719909395.git.leon@kernel.org> <20240703054238.GA25366@lst.de> <20240703105253.GA95824@unreal> <20240703143530.GA30857@lst.de> <20240703155114.GB95824@unreal> <20240704074855.GA26913@lst.de> <20240708165238.GE14050@ziepe.ca> <20240709061721.GA16180@lst.de> <20240709185315.GM14050@ziepe.ca>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Tue, Jul 09, 2024 at 03:53:15PM -0300, Jason Gunthorpe wrote:
> > That whole thing of course opens the question if we want a pure
> > in-memory version of the dma_addr_t/len tuple.  IMHO that is the best
> > way to migrate and allows to share code easily.  We can look into ways
> > to avoiding that more for drivers that care, but most drivers are
> > probably best serve with it to keep the code simple and make the
> > conversion easier.
> 
> My feeling has been that this RFC is the low level interface and we
> can bring our own data structure on top.
>
> It would probably make sense to build a scatterlist v2 on top of this
> that has an in-memory dma_addr_t/len list close to today

Yes, the usage of the dma_vec would be in a higher layer.  But I'd
really like to see it from the beginning.

> . Yes it costs
> a memory allocation, or a larger initial allocation, but many places
> may not really care. Block drivers have always allocated a SGL, for
> instance.

Except for those optimizing for snall transfer of a single segment
(like nvme). 

> My main take away was that we should make the dma_ops interface
> simpler and more general so we can have this choice instead of welding
> a single datastructure through everything.

Yes, I don't think the dma_vec should be the low-level interface.
I think a low-level interface based on physical address is the right
one.  I'll see what I can do to move the single segment map interface
to be physical address based instead of page based so that we can
unify them.