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

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Date:   Thu, 20 Dec 2018 07:36:46 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Noralf Trønnes <noralf@...nnes.org>
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,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

On Tue, Dec 18, 2018 at 08:20:22PM +0100, Noralf Trønnes wrote:
> > +	if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents,
> > +			DMA_BIDIRECTIONAL)) {
> 
> 
> Are you using the DMA streaming API as a way to flush the caches?

This looks rather broken.  Please send the whole patch series to
the iommu list for proper review.

> Does this mean that GFP_USER isn't making the buffer coherent?

How could GFP_USER make memory coherent in any way?