From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH v2] drm/xen-front: Make shmem backed display buffer
 coherent
Date: Tue, 15 Jan 2019 22:36:31 -0800
Message-ID: <20190116063631.GA10881@infradead.org>
References: <20190115140420.4652-1-andr2000@gmail.com>
 <20190116063002.jq3aas6ofhaogmyk@sirius.home.kraxel.org>
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To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Oleksandr Andrushchenko <andr2000@gmail.com>, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, daniel.vetter@intel.com, jgross@suse.com, boris.ostrovsky@oracle.com, noralf@tronnes.org, hch@infradead.org, Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
List-Id: dri-devel@lists.freedesktop.org

On Wed, Jan 16, 2019 at 07:30:02AM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > +	if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents,
> > +			DMA_BIDIRECTIONAL)) {
> > +		ret = -EFAULT;
> > +		goto fail_free_sgt;
> > +	}
> 
> Hmm, so it seems the arm guys could not come up with a suggestion how to
> solve that one in a better way.  Ok, lets go with this then.
> 
> But didn't we agree that this deserves a comment exmplaining the purpose
> of the dma_map_sg() call is to flush caches and that there is no actual
> DMA happening here?

Using a dma mapping call to flush caches is complete no-go.  But the
real question is why you'd even want to flush cashes if you do not
want a dma mapping?

This whole issue keeps getting more and more confusing.