linux-kernel - Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

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Message-ID: <20211007174822.GK2744544@nvidia.com>
Date:   Thu, 7 Oct 2021 14:48:22 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Barry Song <21cnbao@...il.com>, iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>, mike.campin@...el.com,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

On Thu, Oct 07, 2021 at 10:50:10AM -0700, Jacob Pan wrote:

> On platforms that are DMA snooped, this barrier is not needed. But I think
> your point is that once we convert to DMA API, the sync/barrier is covered
> by DMA APIs if !dev_is_dma_coherent(dev). Then all archs are good.

No.. my point is that a CPU store release is not necessary a DMA
visiable event on all platforms and things like dma_wmb/rmb() may
still be necessary. This all needs to be architected before anyone
starts writing drivers that assume a coherent DMA model without using
a coherent DMA allocation.

Jason