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

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Date:   Thu, 20 Dec 2018 09:39:50 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Oleksandr Andrushchenko <andr2000@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Noralf Trønnes <noralf@...nnes.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, daniel.vetter@...el.com,
        jgross@...e.com, boris.ostrovsky@...cle.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

On Thu, Dec 20, 2018 at 05:49:39PM +0200, Oleksandr Andrushchenko wrote:
> This is the only patch [1], no series. And at the moment I think
> there is nothing to review as I am not sure how to deal with those
> shmem pages: this patch is rather to start a discussion on how shmem
> pages can be flushed on ARM (the only workaround I have so far is
> in this patch which uses DMA API). This is where I am looking for
> some advice, so I can implement the patch the right way.

shmem is basically page cache.  So you need to use the DMA streaming
API (dma_map_*) to map it for DMA.  You need to make sure no one
access the kernel mapping at the same time as you do DMA to it,
so the pages should be locked.  This is how the normal file system
I/O path works.