linux-kernel - Re: [RFC PATCH v1 18/18] nvme-pci: use new dma API

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Message-ID: <20240705184846.GF95824@unreal>
Date: Fri, 5 Jul 2024 21:48:46 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Robin Murphy <robin.murphy@....com>, Jens Axboe <axboe@...nel.dk>,
	Jason Gunthorpe <jgg@...pe.ca>, 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 18/18] nvme-pci: use new dma API

On Fri, Jul 05, 2024 at 07:58:06AM +0200, Christoph Hellwig wrote:
> > This is exactly how dma_map_sg() works.
> 
> Which dma_map_sg?  swiotlb handling is implemented in the underlying
> ops, dma-direct and dma-iommu specifically.
> 
> dma-direct just iterates over the entries and calls dma_direct_map_page,
> which does a per-entry decision to bounce based on
> is_swiotlb_force_bounce, dma_capable and dma_kmalloc_needs_bounce.

dma-direct is not going to have "use_iova" flag. Robin pointed to
dma-iommu path.

In that case the flow is dma_map_sg()->iommu_dma_map_sg()->dev_use_sg_swiotlb().

Thanks

> 
>