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Date:   Thu, 29 Nov 2018 19:35:50 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Brian Starkey <Brian.Starkey@arm.com>
Cc:     Christoph Hellwig <hch@lst.de>, "Clark, Rob" <robdclark@gmail.com>,
        Dave Airlie <airlied@linux.ie>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Tomasz Figa <tfiga@chromium.org>,
        Sean Paul <seanpaul@chromium.org>,
        "vivek.gautam@codeaurora.org" <vivek.gautam@codeaurora.org>,
        freedreno <freedreno@lists.freedesktop.org>,
        Robin Murphy <Robin.Murphy@arm.com>, nd <nd@arm.com>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*
Message-ID: <20181129183550.GD30281@lst.de>
References: <20181129140315.28476-1-vivek.gautam@codeaurora.org> <20181129141429.GA22638@lst.de> <CAF6AEGvsFbXGkC+=4oq1xXnoC2EVX2J0k-GqShefcek1=uw4Lw@mail.gmail.com> <CAKMK7uHOKtX2a12itSA0sDg=b77F+0vnLAdEvc_aNGRJTh85=Q@mail.gmail.com> <20181129155758.GC26537@lst.de> <20181129162807.GL21184@phenom.ffwll.local> <20181129165715.GA27786@lst.de> <20181129173301.rye77slpy27regi5@DESKTOP-E1NTVVP.localdomain>
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On Thu, Nov 29, 2018 at 05:33:03PM +0000, Brian Starkey wrote:
> This sounds very useful for ion, to avoid CPU cache maintenance as
> long as the buffer stays in device-land.
> 
> One question though: How would you determine "the last user to unmap"
> to know when to do the final "make visible to CPU" step?

I'd assume the user of the DMA API keeps track of it.  E.g. dmabuf
would be a pretty natural layer to implement common logic for this.