linux-kernel - Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

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Message-ID: <20181220183311.GA30801@infradead.org>
Date:   Thu, 20 Dec 2018 10:33:11 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Oleksandr Andrushchenko <andr2000@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com
Subject: Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

On Thu, Dec 20, 2018 at 07:29:37PM +0100, Daniel Vetter wrote:
> What we most definitely not want to end up with though is actually
> streaming dma, because with that all the zero copy buffer sharing
> tricks become pointless. There's pretty epic amounts of hacks to work
> around this, I have no idea what's supposed to give here.

Err, with streaming DMA buffer sharing is trivial.  The coherent DMA
allocator is what causes all kinds of horrible hacks that can't actually
work on various platforms.