linux-kernel - Re: [PATCH v6 2/2] habanalabs: add support for dma-buf exporter

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Date:   Fri, 1 Oct 2021 11:52:09 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Oded Gabbay <ogabbay@...nel.org>
Cc:     Christian König <christian.koenig@....com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Gal Pressman <galpress@...zon.com>,
        Yossi Leybovich <sleybo@...zon.com>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Doug Ledford <dledford@...hat.com>,
        Dave Airlie <airlied@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        Christoph Hellwig <hch@....de>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, Tomer Tayar <ttayar@...ana.ai>
Subject: Re: [PATCH v6 2/2] habanalabs: add support for dma-buf exporter

On Thu, Sep 30, 2021 at 03:46:35PM +0300, Oded Gabbay wrote:

> After reading the kernel iommu code, I think this is not relevant
> here, and I'll add a comment appropriately but I'll also write it
> here, and please correct me if my understanding is wrong.
> 
> The memory behind this specific dma-buf has *always* resided on the
> device itself, i.e. it lives only in the 'device' domain (after all,
> it maps a PCI bar address which points to the device memory).
> Therefore, it was never in the 'CPU' domain and hence, there is no
> need to perform a sync of the memory to the CPU's cache, as it was
> never inside that cache to begin with.
> 
> This is not the same case as with regular memory which is dma-mapped
> and then copied into the device using a dma engine. In that case,
> the memory started in the 'CPU' domain and moved to the 'device'
> domain. When it is unmapped it will indeed be recycled to be used
> for another purpose and therefore we need to sync the CPU cache.
> 
> Is my understanding correct ?

It makes sense to me

Jason