linux-kernel - Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API

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Message-ID: <20240710062704.GA25953@lst.de>
Date: Wed, 10 Jul 2024 08:27:04 +0200
From: Christoph Hellwig <hch@....de>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Christoph Hellwig <hch@....de>, Leon Romanovsky <leon@...nel.org>,
	Jens Axboe <axboe@...nel.dk>, Robin Murphy <robin.murphy@....com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Keith Busch <kbusch@...nel.org>, "Zeng, Oak" <oak.zeng@...el.com>,
	Chaitanya Kulkarni <kch@...dia.com>,
	Sagi Grimberg <sagi@...mberg.me>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rdma@...r.kernel.org, iommu@...ts.linux.dev,
	linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API

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.