linux-kernel - Re: [Linaro-mm-sig] noveau vs arm dma ops

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Message-ID: <20180426092422.GA26825@infradead.org>
Date:   Thu, 26 Apr 2018 02:24:22 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Jerome Glisse <jglisse@...hat.com>,
        iommu@...ts.linux-foundation.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Thierry Reding <treding@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Christian König <christian.koenig@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] noveau vs arm dma ops

On Thu, Apr 26, 2018 at 11:20:44AM +0200, Daniel Vetter wrote:
> The above is already what we're implementing in i915, at least
> conceptually (it all boils down to clflush instructions because those
> both invalidate and flush).

The clwb instruction that just writes back dirty cache lines might
be very interesting for the x86 non-coherent dma case.  A lot of
architectures use their equivalent to prepare to to device transfers.

> One architectural guarantee we're exploiting is that prefetched (and
> hence non-dirty) cachelines will never get written back, but dropped
> instead.

And to make this work you'll need exactly this guarantee.