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

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Message-ID: <CAKMK7uFxU9Q5u9TxxHqAgFZuQFCFDvxWHDtRLiQf+3hu_4jPcQ@mail.gmail.com>
Date:   Thu, 20 Dec 2018 19:35:15 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     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 7:33 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> 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.

Hm, I thought the streaming dma api is the one that causes bounce
buffers and all that fun. If you're unlucky at least.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch