linux-kernel - Re: [PATCH v2] drm/xen-front: Make shmem backed display buffer coherent

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Date:   Tue, 15 Jan 2019 22:36:31 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Oleksandr Andrushchenko <andr2000@...il.com>,
        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, noralf@...nnes.org,
        hch@...radead.org,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH v2] drm/xen-front: Make shmem backed display buffer
 coherent

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.