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

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Message-ID: <20240712045422.GA4774@lst.de>
Date: Fri, 12 Jul 2024 06:54:22 +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 Thu, Jul 11, 2024 at 08:29:17PM -0300, Jason Gunthorpe wrote:
> So this little detail needs to be taken care of somehow as well, and I
> didn't see it in this RFC.

Yes.  Same about generally not mixing P2P and non-P2P.

> 
> > For the block layer just having one kind per BIO is fine right now,
> > although I could see use cases where people would want to combine
> > them.  We can probably defer that until it is needed, though.
> 
> Do you have an application in mind that would want multi-kind per BIO?

If you are doing say garbage collection in a file system, and do write
that sources data from multiple devices, where some sit directly on the
root port and others behind a switch.

This is all purely hypothetical, and I'm happy to just check for it
and reject it for it now.